AbstractProton spin diffusion measurements were made using the Goldman—Shen technique of bisphenol‐A polycarbonate containing 10 wt% trioctyl phosphate at temperatures below the glass transition temperature. The proton lineshapes reveal the existence of dynamic heterogeneities in the glass and spin diffusion can be observed from the more mobile to the less mobile domain. The time‐scale for spin diffusion is 4–10 ms and the mobile domain size is estimated at 11 Å at 339 K. This estimate is consistent with a mobile domain of approximately two diluent molecules and 11 polymer repeat units. This grouping can be compared with the results of a statistical lattice model used to estimate local environments at this com