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US EPA federal equivalent method (FEM), the tapered element oscillating microbalance (TEOM) (Rupprecht and Pattachnick, Albany, NY) for measuring continuous hourly PM₁₀, was collocated with a filter-based federal reference method (FRM), the PM₁₀ sequential filter sampler (SFS), at five sites in Mexico City during February and March, 1997. A PM₁₀ mass comparison showed significant differences between instruments that exceeded the expected uncertainties. In general, the TEOM measured higher and more variable PM₁₀ than the SFS. It was found that when averaging 24h of PM₁₀ concentrations, exposures to very high levels were missed. The TEOM measurements exhibited much less spatial variations across different sites than the similar comparison of SFS measurements. The mass and chemical composition of the SFS measurements showed that the majority of the PM₁₀ mass can be explained by …