As part of the MIRAGE (MILAGRO) study conducted 7–30 March 2006 in Mexico City and its Metropolitan Area (MCMA), fine particulate matter (PM 2.5 ) was collected using two Tapered Element Oscillating Microbalance (TEOM) systems, and a Partisol instrument at the T1 super-site (Tecamac, State of Mexico). Inorganic analysis was performed on filter-based (PM 1 , PM 2.5 -URG) measurements also collected at this site. Measurements from the gravimetric (TEOMs, Partisol) and URG systems were inter-compared with chemical speciation measurements using a Particle Into Liquid Sampler (PILS) and Thermal Optical methods.Mass and chemical balances applied over the first part of the study (11–22 March) showed that a TEOM using a device (SES) which reduces particle-bound water and retains a fraction of semi-volatile compounds (SVM) gives readings ∼30% larger than a conventional TEOM. In the second part of the study (26–30 March), the loss of SVM during TEOM-heated filter collection (both systems) represented a significant fraction of PM 2.5 mass due to changes in particle composition. Overall, when nonvolatile nitrate dominated (i.e., when associated with crustal species and not NH 4 + ) and/or sulfate dominates (SO 4 2− /NO 3 − molar ratio is >1), PM 2.5 mass readings are in agreement with those reported for the T1 site if TEOM is using a SES device. However, when volatile nitrate dominates (i.e., NH 4 NO 3 ) or SO 4 2− /NO 3 − molar ratio is <1, a larger fraction is lost from both TEOMs (with or without the SES device). Under the latter regime, uncertainties are large and gravimetric losses may reach 30%–50%. The gravimetric PARTISOL instrument recorded lower readings under all of the aforementioned conditions with differences versus TEOMs decreasing with increasing RH. These findings call for a careful characterization of such volatilization biases to improve current PM (PM 10 , PM 2.5 ) measurements/networks, especially in alkaline-rich environments that can favor such biases.With regards to PM 1 and PM 2.5 filter-based measurements, findings are: 1) crustal-related elements are important features in the PM 2.5–1 size fraction; 2) a factor of ∼2 overestimation of SO 4 2− concentrations is recorded on substrates during PM collection and 3) main elements of a typical urban aerosol size distribution are concentrated in the 1μm (versus 2.5μm) size fraction.