We present a comparative core level photoemission study of the interaction between As and the (110) surfaces of GaAs and InP. In both cases it is found that As forms well ordered overlayers with the periodicity of the substrate surface. The interactions in the two cases are, nonetheless, very different. The bonding at GaAs is very weak, and the adsorbed species desorbs below 100 degrees C. in contrast, the interaction with InP(110) is significantly stronger, and at temperatures around 330 degrees C an As-P exchange reaction is indicated and is seen the core level spectra. The latter data are interpreted in terms of a recently proposed structure model involving an As-covered InAs layer.