Building on a recently introduced technique for continuous online monitoring of absolute weight-average molecular weight M-w of polymers during polymerization reactions, a method is introduced here for finding a useful approximation to the polymer mass distribution and moments at each instant during the reaction. The method is based on finding the instantaneous weight-average mass at each instant, M-w,M-inst, from an expression involving the derivative of the cumulative M-w, which is directly monitored online. The method is restricted to chain growth polymerization where individual chains are produced and terminate rapidly compared to the overall time it takes for complete monomer conversion to polymer. Data from two different polymerization experiments that were monitored online were then analyzed. One of these experiments is from a reaction for poly(vinylpyrrolidone), which produced a bimodal population, and the other from a reaction that produced a broad, unimodal distribution. Finally, other possibilities for online polydispersity measurements are briefly discussed.