A method of introducing an additive, for example a dye, into a polymeric material comprises using a liquid formulation comprising the additive, a vehicle and an active compound added to increase the melt viscosity of the polymeric material. The active compound may be a multi-functional anhydride. The liquid formulation is suitably contacted with the polymeric material in a melt processing apparatus and suitably a cavity transfer mixer is used in the process. A fiber is suitably subsequently produced.