Why is kerosene stopping my red LEDs from illuminating?
We measured the forward voltage and didn't notice any change. 


Physically, I'm pretty sure this means the semiconductor interface is still producing photons at the same rate and wavelength as before.

So, something happens to those photons.

What you should do is get a working source of red light of the same wavelength (e.g. another one of your LEDs), extract the "lens" material from a "donor" LED:



e.g. by cutting it off with a razor blade, testing transmission of red light before and after having soaked that material in kerosene.

Since that lens is tiny, you should probably use something like a piece of cardboard with a hole punched through it with some kind of needle (don't let the hole get to small, lest you want to have much diffraction...) and put the lens in front of that hole.

My guess is that soaking the material in kerosene leads to a drastic change in optic properties, and that might very well mean that either


your lens is now absorbing red light or
your lens is now not focussing red light, but spreading it.


To rule out 2., you'd need a very very dark room and some way to guesstimeasure the distribution  of light. So in effect, without optical design lab equipment, either way, Kerosene contains a mixture of different hydrocarbons, and those are soluble in other hydrocarbons, such as the transparent material used to protect the actual LEDs and act as a lens.