The advent of automatic sorption isotherm instruments has opened the way for routine use of gas adsortion analyses in the measurement of the ultrastructural characteristics of gels. The purpose of this paper is a comparison of the three automatic pore size measuring instruments available on the market in the US. Three silica gels, made in identical fashion, were analyzed, one on each instrument, and the results compared. The influence of degassing variables and sample inhomogeneity on sample analysis were also examined. See also <i>AATA* 3 23-2932</i> for reference to entire book. -- AATA