Using an in situ stress measurement technique that measures stress as a function of annealing temperature, instabilities in mechanical stress induced by heat treatment in a variety of doped/undoped SiO2 films deposited by atmospheric‐pressure chemical vapor deposition (APCVD), low‐pressure chemical vapor deposition (LPCVD), and plasma‐enhanced chemical vapor deposition (PECVD) techniques have been investigated. A large hysteresis in mechanical stress, caused by first heat treatment to which the as‐deposited films are subjected, has been observed in films deposited by APCVD/LPCVD techniques. No such hysteresis is observed in films deposited by PECVD technique. Hysteresis in APCVD/LPCVD films is found to vanish once the films are heat treated at or above 800 °C. Stress behavior of heat treated (≥800 °C) films is found to resemble that of thermally grown SiO2 films and can be described by simple elastic theory. During the first annealing cycle, APCVD/LPCVD films have been found to develop stress levels as large as an order of magnitude higher than their as‐deposited stress value. Maximum stress developed in the film during heat treatment and the temperature TR at which maximum stress occurs have been found to be dependent on phosphorous content in the film. For heat treatment below this temperature TR, the stress on cooling becomes significantly larger than the as‐deposited stress. The results are discussed in terms of oxide densification, the presence of hydrogenous species, and phosphorous.