expert in/on

I'm not sure about that.

I would always say "expert in".

I'm pretty sure there is no rule, though, it's just personal choice.

I think either is fine but I feel there is a minor shading of difference.

You tend to be an expert in 'a field of study'
but an expert on 'a particular topic within a field of study'

Or maybe it's an involvement/action/immersion thing

An 'expert on' tends to know a lot about the thing

but an 'expert in' tends to be very good at doing the thing eg sexing chickens, karate et al

maybe it's the same shade of grey.

ThinK:


expert (in the subject matter of . . . .)

or

expert on (subject)


Expert in nuclear dynamics.

Expert on sports.
Expert in running.

. . . and that's nuculear.

Is it?

I see you have a sense of humour, Sullyfish, or else you come from Texas? Please clarify!

I would write that a person is

an expert in the field/subject/area of [a subject]

or

an expert on [something specific]