The present article analyses the evolution of the concentration of the Colombian electrical generation market based on the technological diversification indexes and H-H and C-4 concentration indexes, during the period 1995-2004. Concentration herein is an enterprise behavior which, from the regulator's perspective, can attempt against efficiency results expected upon reformation of the industry's rules of the game. The conclusion after the analysis, on the one hand, is that the concentration levels have decreased in the study period, passing from a strong oligopoly to a moderate one; on the second hand, indexes are those with high technological diversification.