Semi‐insulating polycrystalline silicon (SIPOS) can be used as part of a passivation scheme on semiconductor devices. During processing the film experiences temperatures several hundred degrees higher than the SIPOS deposition temperature of 640°C. We have investigated the effect of high temperature anneals on the physical properties of SIPOS films containing 25 atomic percent oxygen. Heat‐treatments of 900°C or greater in inert ambients densify the film resulting in a film thickness decrease (densification) on the order of 10% and decrease the etch rate of the film by a factor of 6 in a SIPOS etch solution. Also, the infrared transmission spectrum of the SIPOS film changes as a result of annealing. Similar infrared changes in films have been interpreted to be due to a relaxation of bond strain and a decrease in film porosity. Film stress has also been investigated before and after annealing and was found to change from tensile to compressive. This is consistent with the interpretation of the infrared data.