Enthalpy probe measurements in supersonic plasma flows are subject to various sources of error which are difficult to quantify experimentally. The relative importance of several such errors has been assessed by means of detailed two-dimensional numerical simulations of high-speed plasma flow impinging on an enthalpy probe. The simulations show that moderate uncertainties in upstream pressure and composition (i.e., degree of ionization) can lend to significant errors in the velocity and temperature inferred from the measurements. These errors tend to be larger irt velocity than temperature. A second potential source of error is that enthalpy probe data are generally interpreted by means of simplified analytical relations which neglect the effects of finite-rate ionization, internal electronic excitation, thermal radiation, probe cooling, and probe sampling. The importance of these effects was also assessed, and the resulting errors were not significant under the conditions examined. We conclude that enthalpy probe measurements in supersonic plasma flows are useful in situations where the upstream pressure and degree of ionization are known to reasonable accuracy.