Despite the widespread application and use of velocity-measuring instruments, there are insufficient data available on the influence of density variation in an air flow field on velocity measurements by common instruments such as the rotating vane anemometer (RVA), the deflecting vane anemometer (DVA), and the hot-wire or heated-probe thermal anemometer (TA). Density variations in the flow field may significantly affect the output of these anemometers. Reports on the theoretical phase of ASHRAE Research Project 698, aimed at developing correction factors to reduce such measurement errors. Theoretical predictions of velocity variations as a function of flow-field temperature, pressure, humidity, and/or density have been developed for both vane and thermal anemometers. Results indicate significant effects of temperature and pressure on instrument output. In the case of the vane anemometer, an overall correction for fluid conditions in the flow field may be based on a density correction factor. However, it must be noted that secondary effects of temperature and humidity on the lubricants and bearings of a vane anemometer may affect the output of the instrument. For the thermal anemometer, other applicable thermophysical properties of the fluid (i.e., viscosity, thermal conductivity, and specific heat) rule out a correction factor based on density alone.