Previous findings that related diversification creates value have been called into question over concerns about methodology and measures. Reviewing existing theory to consider how a firm's knowledge base interacts with its product market activity, I address several of these concerns by creating a measure of technological diversity based on citation-weighted patents. The measure indicates a firm's opportunity for corporate diversification based on economies of scope in valuable knowledge assets, is defined for both single- and multibusiness firms, and is not correlated with more fundamental aspects of diversification, such as the number of businesses in the corporate portfolio. Evidence from a large sample of firms shows the positive relationship between diversification based on technological diversity and market-based measures of performance, controlling for R&D intensity and capital intensity as further indicators of the type of assets underlying diversification. Results hold when controlling for the endogeneity of diversification and performance in a cross-sectional sample or when controlling for unobserved factors using panel data. Copyright © 2006 John Wiley & Sons, Ltd.