The aim of this study is to provide new theoretical insights and empirical evidence on the effect of market and technological uncertainty on the market valuation of a firm's R&D capital. A set of hypotheses is developed adopting a real options logic and tested on a panel dataset of 290 manufacturing firms traded in the UK. Consistently with our theoretical model, we show that market and technological uncertainty have distinct effects on the valuation of R&D investments. The results have several important implications for resource allocation to R&D under uncertainty, which we discuss in the concluding section. Copyright © 2008 John Wiley & Sons, Ltd.