A modified redox system intended for initiation of radical polymerization of unsaturated polyester resins was investigated. The main feature of this modification was the presence of an additional accelerator-sodium formaldehyde sulfoxylate (also known as 'Rongalit'). Rongalit acts as a reducer in hydrophilic medium. The interaction between Rongalit and the hydrophobic ingredients of the reactive mixture occur at the boundary surfaces between the different phases. The specific influence of the initiating system's components over the polymerization kinetics was estimated. The technological time needed for gelling and then - for complete hardening of the resin was determined. The end properties of the cured polyesters as well as those of their derivative glass-fibres-reinforced composites were not found to be dependent on the mode of initiation of the polymerization process.