Mechanisms underlying the refractive index changes in germanosilica films deposited by hollow cathode plasma enhanced chemical vapor deposition and subjected to UV irradiation are proposed based on observed changes in film thickness, stress, and structure. An increase in refractive index after UV exposure is observed in films deposited under low ion bombardment conditions. This increase in refractive index is accompanied by a reduction in film thickness which is an order of magnitude larger than that expected from the Lorentz–Lorentz relation. This behavior is shown to result from: (i) a significant degree of porosity in the as-deposited material; (ii) oxygen deficiency of the as-deposited material. Upon UV irradiation, the porous structure is compacted, thus accounting for the large decrease in thickness, while the oxygen deficiency is reduced causing a decrease in the material polarizability and counteracting the effect of the thickness reduction. On the other hand, germanosilica deposited under high ion-bombardment conditions is of normal optical quality and exhibits a decrease in refractive index after exposure to UV. This refractive index reduction is shown to be the result of three processes: structural dilation and stress relief on one side; and an increase in material polarizability on the other, with structural dilation having the largest effect. Annealing of the exposed samples has shown that most of the polarizability increase is likely to be annealed out at 500 °C, while the refractive index change caused by structural dilation is stable up to 800 °C. Finally, it is shown that during plasma enhanced chemical vapor deposition, germanosilica is more prone to nucleation and columnar growth than pure silica and therefore a higher level of ion bombardment is required in the former case in order to obtain a high quality homogeneous material. ©1997 American Institute of Physics.