Thin films of binary borosilicate and phosphosilicate glasses of composition varying from one end‐component to the other can be deposited by reacting Ar‐diluted mixtures of B2H6‐SiH4O2 and PH3‐SiH4‐O2 on heated Si substrates at 300–450 °C. Room temperature infrared transmission spectra in the range 4000−250 cm−1 of these vapor‐deposited dielectric glass films have been studied systematically as a function of film composition. A detailed analysis shows that compositional dependence of the intensity of the bands associated with a B‐O‐Si mode in the borosilicate and a P‐O‐Si mode in the phosphosilicate system conforms to a simple random network model consisting of (1) Si in 4‐fold coordination with oxygens, all of which are bridging a pair of Si and/or B (P) atoms; (b) B in 3‐fold coordination with O and (c) P in 4‐fold coordination with oxygens, one of which is doubly bonded to form a terminal P=O.