Most industrialized countries apply special tax incentives to boost the R&D expenditures of firms. This study considers the design of such R&D tax incentives as applied in the European Union and simulates its effect on the post-tax R&D expenditures of firms in different industries and different profit/loss-situations by means of the simulation model European Tax Analyzer. Any restrictions and progressive tax incentives are explicitly taken into account. Our results indicate that for designing and measuring public support to R&D it is often not sufficient to focus only on tax rate effects of R&D tax incentives and the design of a tax incentive must be in accordance with the framing tax system in order to be effective. As soon as there are any limitations in place, our results suggest a considerably lower impact of R&D tax incentives on the post-tax R&D expenditures than the commonly used B-Index by the OECD. The results clearly illustrate the beneficial impact of immediate cash refunds for unused tax incentives.