The interdiffusion of polymer chains during film formation from direct mini-emulsified deuterated and protonated polystyrenes (DPS = mol wt 150 000 and 185 000; HPS = mol wt 150 000 and 200 000) was characterized by small-angle neutron scattering (SANS) and tensile strength measurements. Films containing 6 mol % deuterated particles were annealed at several temperatures above T-g for various periods of time. The average interpenetration depth of deuterated polystyrene chains depended on the one-fourth power of the annealing time up to the reptation time, tau, shifting to the one-half power after the reptation time. The center of mass diffusion coefficients from the SANS data were 2.4 x 10(-16) cm(2)/s for M(2) = 150 000 and 1.5 x 10(-16) cm(2)/s for M(n) = 185 000 at an annealing temperature of 135 degrees C. The diffusion activation energies of the present system were 52 +/- 4 kcal/mol for the temperature range 125 less than or equal to T less than or equal to 155 degrees C. Full tensile strength was achieved at a penetration depth of 90-100 Angstrom, which is comparable to 0.81 times the radius of gyration of the whole polystyrene chain (86 Angstrom for M(n) 150 000) predicted theoretically. Finally, latex samples containing 1.5 wt % cosurfactants showed a faster interdiffusion rate especially at t > tau and a smaller activation energy than the corresponding pure latex sample.