The radical polymerization of C12-C18 alkylmethacrylates in solution of mineral oil has been studied in semibatch condition by using 2,2’-azobis (2-methylbutyrronitrile) as initiator. Many kinetic runs of polymerization have been performed in different operative conditions. A kinetic model has been developed to interpret and simulate these runs. C12-C18 alkylmethacrylate polymerization in mineral oil concentrated solution occurs through a classical radical chain mechanism. Kinetic parameters for the initiator decomposition and for the radical chain propagation rate resulted the same as reported by the literature. A particular behavior of this system is the intervention of the gel effect observed from the beginning of the reaction as a consequence of the high initial viscosity.