A unified methodology is developed to calculate the fraction of initiator derived radicals that initiates the polymerization, i.e. the initiator efficiency, as a function of polymerization time and polymerization conditions. In the presented approach the most important reactions that initiator derived radicals undergo are taken into account. All involved parameters have a clear physical and fundamental meaning and no adjustment to experimental data is required. The presented methodology is applied to diacyl peroxide initiators that are commonly used in industrial vinyl chloride suspension polymerization processes: dodecanoyl peroxide and benzoyl peroxide. Validation is performed by comparing calculated and experimental data for the monomer conversion and averages of the molar mass distribution within a polymerization temperature range of 323-333 K and an initiator concentration range of 0.26-4.2 wt% based on the monomer. (C) 2009 Elsevier B.V. All rights reserved.