Calibration of a Fenn‐type nozzle beam source and the limitations due to background attenuation, skimmer interference, and condensation are discussed. The nozzle flow rateNnis calculated, and the peaking factor &kgr; is determined from both radial pressure surveys and effusive‐to‐supersonic transition measurements. Stage pressure measurements verify bothNnand &kgr;. These quantities specify the ideal beam flux in the absence of attenuation, interference, or condensation. Background attentuation depends on the effective scattering cross section, which can be quite large for finely collimated beams. Serious skimmer interference occurs below a critical value of the skimmer Knudsen number and depends on individual skimmer details. Condensation is observed and found to be predictable according to the known scaling laws. A calculation of absolute beam fluxes is presented. Nonideal behavior of the speed ratio and average particle velocity are also examined. Data are given for the gases H2, He, Ne, and Ar.