Recently SAMPSON (1975) has shown that the mean function is enough to characterize the one parameter exponential family. In this paper we consider the class of modified power series distributions introduced by the author (1974) and exhibit a general method of obtaining the distribution if the ratio variance/mean is known. As illustrations characterizations of the POISSON and the geometric distributions are obtained. Considering the continuous version of the exponential family, it is shown that the equality of mean and standard deviation characterizes the exponential distribution.