The number of empirical studies on informal contacts in the field of innovation is still very low due to the lack of variables able to identify and estimate the knowledge transfer through informal relationships among individuals. As a way to overcome this lack of measurement tools, we propose a analysis based on variables found in the Community Innovation Survey. The application of our methodology to a sample of service innovating firms, using the Spanish Technological Innovation Panel (PITEC) as data source led us to identify two groups of activities with a high propensity to establish informal contacts with customers and suppliers. This result has also allowed us to recognize external flows of information in low technology-intensive sectors; this evidence could hardly be observed through traditional indicators of cooperation due to the techno-scientific nature and formal, structured character of the partnership represented in these variables.