This invention relates to a method for the determination of the average particle size or particle size distribution of a material in a gas phase reactor comprising: 1) analyzing the average particle size and particle size distribution of a baseline composition using the method described in ASTM D1921; 2) analyzing the average particle size and particle size distribution of said baseline composition using an FT-NIR analysis technique; 3) preparing a calibration matrix by comparing results from said reference analytical technique to the results from said FT-NIR analysis technique; 4) analyzing the material using an FT-NIR technique; and 5) identifying and quantifying the type and content of particles present in the material by comparing spectral data obtained from said FT-NIR technique of the material to said calibration matrix.This invention also relates to a process for determining polymer properties in a polymerization reactor system using such techniques.