The two words get used as though they were the same thing, including by people who should know better. They are not, and the gap between them is where a claim either holds up or falls over.
Here is the short version. Most sustainable aviation fuel in the market today is a biofuel. Very little biofuel is sustainable aviation fuel. And some SAF is not a biofuel at all.
Is sustainable aviation fuel a biofuel?
Usually, yes. The dominant production route today is HEFA, which stands for Hydroprocessed Esters and Fatty Acids, and it converts used cooking oil, animal fats, tallow and technical corn oil into jet fuel. All of that is biomass, so the resulting fuel is a biofuel in the ordinary sense of the word.
So if someone calls SAF a biofuel, they are usually describing it accurately. The trouble starts when the sentence runs the other way.
Why isn’t all biofuel sustainable aviation fuel?
Two reasons, and they are worth separating.
The first is simply what the fuel is for. Biofuel is a huge category that includes road diesel, ethanol blended into petrol, marine fuel and heating fuel. Only a small part of it is refined to jet specification, and jet specification is unforgiving because the fuel has to behave identically to fossil kerosene at altitude.
The second reason is the one that matters commercially. “Biofuel” describes where a fuel came from, and says nothing at all about whether it was produced responsibly. A fuel can be entirely biomass-derived and still be tied to land use you would not want your name against. Certification is what closes that gap: a recognised scheme assesses the feedstock, the production pathway and the chain of custody, and only then does a batch earn a verified sustainability claim.
That is the whole distinction. Biofuel is a description of origin. Sustainable aviation fuel is a description of origin that somebody independent has checked.
Is there SAF that isn’t a biofuel?
Yes, and it is the clearest way to see why the words are not interchangeable.
e-SAF, also called power-to-liquid, is made from renewable electricity, water and captured carbon dioxide. There is no biomass anywhere in it. It is unambiguously sustainable aviation fuel and unambiguously not a biofuel. Electricity is the dominant cost, so it depends on cheap renewable power and it is early, but it exists and it breaks the equivalence.
The reverse case is worth knowing too. Lower Carbon Aviation Fuel, or LCAF, is a fossil aviation fuel that meets the sustainability criteria set out under CORSIA. It is neither a biofuel nor a sustainable aviation fuel, despite how close the names sit.
Does the distinction matter for what I can claim?
It decides it.
A reduction claim rests on the certificate behind the batch, and certificates are issued against a scheme’s criteria rather than against a feedstock category. “We bought biofuel” is a purchasing statement. “We retired the certified environmental attributes of a batch audited under RSB or ISCC, and no one else can claim them” is a position you can defend in front of an auditor.
Both descriptions appear in our own certification, which is a useful illustration. Our RSB certificate covers us as a trader of SAF and of maritime biofuels: two different words on one certificate, because they are two different things.
Which word should we use?
Use the one that is true of the fuel in front of you.
For aviation, say sustainable aviation fuel, or SAF once you have glossed it, because that is what the certificate says. For shipping and heavy road freight, biofuel is often the accurate term and there is no reason to avoid it. What does not work is treating them as synonyms, because the moment they become interchangeable in your copy, the certification that separates them stops doing any work.