The purpose of this investigation was to measure the conversion of five inlay materials with different curing methods. In particular, the influence of secondary curing in light ovens—that is, additional curing with light and heat simultaneously—was evaluated. The conversion was determined by infrared multiple internal reflection spectroscopy. The inlays were cured with three curing methods (A, B, and C). The conversion was recorded immediately after ended curing and after an additional storage period (postcuring). The mean conversion with method A, using a handheld curing unit, was 42.1% and increased significantly to a mean of 55.4% during the storage period at (37 ± 1)°C. Both curing methods B and C, including the use of light ovens, gave a mean conversion of 65%, which did not increase significantly during the storage period. This investigation demonstrated that the degree of conversion depends on the curing method used.