In the context of profound economic growth that has been taking place in LatinAmerican countries, the extentof innovation in high technology sectors such as Knowledge Intensive BusinessServices (KIBS), is fundamental toconsolidate a virtuous developing path.In this sense, successful innovation in business depends on the development andintegration of new knowledge in the innovation process. This paper analyzes theexistence of complementarity between internal and external sources of knowledgefor innovation activities infirms from the software sector ofArgentina, taking account of the limited treatment of these matters in studies of innovation of these sectors in emerging economies. The theoretical framework isgrounded on the Make and Buy literature, innovation studies and KIBS literature; and the econometric tools involves a series of estimations of the innovationfunction (Ordered Probit, Tobit and Probit) and super and sub modularity tests.The results allow us to state the existence of complementary relations betweeninternal and external knowledge sources,regardless the regression model considered, which underline the robustness of the quantitative exercise. Also, as a secondary objective, we explore thedegree that complementarity relations are influenced by diverse characteristicsof the firms and contextual aspects