There is growing interest in the Professional service firms because they are seen as archetype of the knowledge-based economy. In this study we look at under researched area of exploitation of synergies in professional service firms and its implications for performance. Overcoming the uni-dimensional nature of extant studies, we examine the performance implications of diversification along the twin dimensions of services they offer and the knowledge of the industry domain of their clients. We hypothesize that moderate levels of coherence in these dimensions lead to improved performance while excess coherence in these domains lead to diminished performance. These predictions are tested and supported by data from the Indian IT industry which is synonymous with emergence of knowledge economy in India. Our study thus contributes to the theory of diversification of professional service firms.