Monitoring of the liquid pressure in a casing-tubing annulus confined by the wellhead at the surface and by a packer above the injection interval is a simple and inexpensive method of detecting the loss of mechanical integrity in wells used for disposal of liquid wastes. However, variations in annular pressure which occur in response to changes in injection rate and injected fluid temperature complicate the interpretation of annular pressure data. The variation of fluid and formation temperatures with depth, the expansion and contraction of fluids and metal tubing with pressure and temperature, and wellbore depth and geometry, have been combined in a model of annular pressure response to fluid injection. The response to changes in injected fluid rate and temperature have been simulated with the model and verified by field experiment. Although the response to a change in injection rate or temperature is greatest in the first two hours following the change, annulus pressure continues to vary with time and a true steady-state was not attained.