The unsteady mass transfer from a contaminated fluid sphere moving in an unbounded fluid is examined numerically for unsteady-state transfer. The effect of the interface contamination and the flow regime on the concentration profiles, inside and outside a fluid sphere, is investigated for different ranges of Reynolds number (0 < Re < 200) and Peclet number (0 < Pe < 10(5)), viscosity ratio between the dispersed phase and the continuous phase (0 < kappa < 10), and the stagnant-cap angle (0 degrees < theta(cap) < 180 degrees). It was found that the stagnant-cap angle significantly influences the mass transfer from the sphere to a surrounding medium. For all Peclet and Reynolds numbers and kappa, the contamination reduces the mass transfer flux. The average Sherwood number increases with an increase of stagnant-cap angle and reaches a maximum equal to the average one for a clean fluid sphere at low viscosity ratio and large Peclet numbers. A predictive equation for the Sherwood number is derived from these numerical results. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 1684-1692, 2011