Fitter ion-interaction parameters far 93 1:1 electrolytes have been calculated by least-squares analysis of published experimental osmotic coefficient data at 25 degrees C. The quality of the resulting fit of these data, expressed as the sum of squared residuals, the standard errors in the least-squares estimates, and the variation of the residuals with electrolyte concentration, is shown to be consistently superior to that obtained with the use of parameters taken from previous publications. The discrepancies between the present estimates of the Fitter parameters and the published values are mainly concentrated in beta((1)). The relative standard error in this parameter depends strongly on the maximum molality represented in the data and is typically at least 1 order of magnitude greater than the relative standard errors in the other parameters. This observation is interpreted by examining the contours of the sum of squared residuals in parameter space. Trends in the quality of fit for different classes of electrolytes are also described.