Calculations of temperatures and supersaturations in nucleation experiments often include the assumption that all gases are ideal and they ignore the increase in vapor pressure due to the presence of noncondensible gases. Regardless of experimental technique, such assumptions can lead to substantial errors when comparing experiment to theory, especially when included in only one or the other. To demonstrate these effects, the procedures for calculating the temperatures and supersaturations in thermal diffusion cloud chambers are examined in detail. Nonideal gas effects do alter them; however, these effects also alter the rates calculated by nucleation theory and the effect is larger on nucleation theory than on the analysis of the experimental data. Due to the presence of noncondensible gases there also is a Poynting type effect, but it very nearly cancels in thermal diffusion cloud chambers when included consistently in both theory and experiment. An additional effect which arises only in nucleation theory is the nonzero compressibility of the critical nuclei. The net result (for n-nonane), when these effects are included consistently in both experiment and theory, is to reduce the temperature dependence of the multiplicative correction to theory necessary for agreement with experiment. None of these effects account for the previously observed apparent decrease in nucleation rate with increasing total pressure. Also critically examined are the uncertainties in the transport properties used to calculate the chamber profiles. Of particular interest is the uncertainty in the thermal diffusion factor which changes systematically with composition and thus with total pressure.