Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Objectives. Stress development during the polymerization process continues to be a major factor that limits predictability and longevity of resin composite restorations. This study evaluated the effect of the photoinitiator type on the maximum rate of polymerization (R(p)(max)), stress development (final stress and maximum rate, R(stress)(max)), degree of conversion (DC) and cross-link density (CLD) of materials containing camphorquinone (CQ), phenylpropanedione (PPD) or CQ/PPD. Materials and methods. R(p)(max) was evaluated via differential scanning calorimetry (DSC). Contraction force measurement was assessed with a single cantilever device for 5 min. The samples were subsequently tested by infrared spectroscopy (FTIR) to evaluate the DC. After, samples were soaked in ethanol to evaluate the swelling coefficient (alpha) as a way to estimate the CLD. The results were analyzed by one-way ANOVA and Tlukey's test (p = 0.05). Results. CQ showed the highest R(p)(max) and R(stress)(max). PPD produced the lowest DC and the highest alpha. The mixture CQ/PPD produced statistically lower Rpmax and R(stress)(max) than CQ alone, but similar DC and CLD. Conclusion. CQ/PPD reduced the R(p)(max) and R(stress)(max), without a reduction in DC and CLD. Therefore, the use of alternative photoinitiator systems could be a promising way to reduce the stress developed during the composite's polymerization without affecting the final properties. (C) 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.255566572Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)CAPES [BEX3667/05-7]