Thin (200 Angstrom) layers of SiO2 were deposited by plasma enhanced chemical vapor deposition onto GaN and InGaP patterned with transmission line measurement contact pads. The sheet resistance of n-GaN and n- and p-InGaP was measured as a function of N2O/SiH4 ratio, rf chuck power, pressure and substrate temperature during the SiO2 deposition, The sheet resistance ratio before and after the deposition varied from 0.7 to 1.2, reflecting a competition between mechanisms that decrease doping (hydrogen passivation, ion-induced deep traps) and those that increase it (creation of shallow donor states in n-type material through preferential ion of the group V elements, gettering of hydrogen in p-type material). Under most conditions, SiO2 deposition creates minimal changes to the electrical properties of GaN and InGaP.