This paper examines the processes and dynamics of ecologically-important inorganic chemical ( primarily NO 3 -N) accumulation and loss in boreal forest snow during the cold winter period at a northern and southern location in the boreal forest of western Canada. Field observations from Inuvik, Northwest Territories and Waskesiu, Saskatchewan, Canada were used to link chemical transformations and physical processes in boreal forest snow. Data on the disposition and overwinter transformation of snow water equivalent, NO À 3 , SO 2À 4 and other major ions were examined. No evidence of enhanced dry deposition of chemical species to intercepted snow was found at either site except where high atmospheric aerosol concentrations prevailed. At Inuvik, concentrations of SO 2À 4 and Cl À were ®ve to six times higher in intercepted snow than in surface snow away from the trees. SO 4 -S and Cl loads at Inuvik were correspondingly enhanced three-fold within the nearest 0 . 5 m to individual tree stems. Measurements of snow aected by canopy interception without rapid sublimation provided no evidence of ion volatilization from intercepted snow. Where intercepted snow sublimation rates were signi®cant, ion loads in sub-canopy snow suggested that NO À 3 volatized with an eciency of about 62% per snow mass sublimated. Extrapolating this measurement from Waskesiu to sublimation losses observed in other southern boreal environments suggests that 19±25% of snow inputs of NO À 3 can be lost during intercepted snow sublimation. The amount of N lost during sublimation may be large in high-snowfall, high N load southern boreal forests (Quebec) where 0 . 42 kg NO 3 -N ha À1 is estimated as a possible seasonal NO À 3 volatilization. The sensitivity of the N ¯uxes to climate and forest canopy variation and implications of the winter N losses for N budgets in the boreal forest are discussed.