In this study, we use panel data to analyse the impact of an R&D tax credit on R&D personnel, particularly the impact on Ph.D. holders allocation, comparing low R&D intensity firms with medium-high and high R&D intensity firms. The results show that, in medium-high and high R&D intensity firms, the R&D tax credit had a significant impact on allocating Ph.D. holders in firms after 3 years of participation in the tax incentive scheme. We use a database covering 7,710 firms that performed R&D at least once in Portugal over the 23-year period 1995 to 2017, provided by the official business R&D survey data and a database of firms that applied for tax credit incentives at least once in the same period. Based on the estimation of impulse-response functions by local projections, we assess the impact of introducing the tax incentive scheme for corporate R&D in firms from different R&D intensity sectors.