Summertime urban PM2.5 was collected on cellulose filters in downtown Toronto, using a customized air sampler (635 l/min). Mass concentrations for up to 19 trace elements/ions were measured by ICP-AES, INAA and IC. Source apportionment was performed on these results including additional carbon and total mass concentrations using positive matrix factorization (PMF). PMF factors exhibited trends that indicated soil (18%), stationary (19%), secondary (48%), and vehicle (15%) sources. Potential source contribution function (PSCF) analysis identified probable sources of the stationary and secondary PM2.5 as originating from the south and southwest of Toronto.