As more plants begin to experiment with chemically enhanced primary treatment (CEPT) and report their results, it is becoming clear that there is considerable variation in the degree of enhancement seen with CEPT at different plants. The primary reason for this is likely the difference in the wastewater characteristics, and in particular, the differences in the particle weight distributions at different plants. There has not been much work done in this area to characterize these differences or to investigate their impact on CEPT. The jar tests that are commonly used to evaluate the efficacy of CEPT provide valuable information on the extent of removal possible; however, they do not help explain why the achievable removals vary from plant to plant. This paper presents a procedure based on particle weight distribution tests that helps explain the rationale behind the removal efficiencies achievable with and without CEPT. The procedure can be used as a predictive and diagnostic tool to evaluate primary treatment performance. This approach was used in a full-scale evaluation of CEPT at the 88-million gallon per day (mgd) Clark County Sanitation District (CCSD) wastewater treatment plant in Las Vegas, Nevada.