Method for treatment of pellets (1) comprising a thermoplastic polymer sheath (2) surrounding glass filaments (3), which glass filaments are covered at least in part with an impregnating agent and extend in a longitudinal direction of the pellets, the method comprising maintaining the pellets for a period of time at an elevated temperature. The method results in an improved impregnation of glass filaments in the pellets resulting in (a) lower amounts of glass separating from the pellets when such are subjected to repetitive mechanical loads and (b) less agglomerations of glass filaments in moulded products manufactured from the pellets.