Reaction pathway investigations of the solvothermal preparation of nanocrystalline CuIn1-xGaxSe2 in triethylenetetramine reveal the early formation of a previously unreported Cu2-xSe(s) intermediate. Over 24 hours, this reacts with In and Se species to form CuInSe2(s). If Ga is present, the reaction proceeds over an additional 48 hours to form CuIn1-xGaxSe 2. Adding ammonium halide salts reduces the CuInSe2 formation time to as little as 30 minutes. It is proposed that in these cases, Cu2-xSe particle growth is limited via a competitive Cu-halide complex formation. The smaller Cu2-xSe particles may react and form CuInSe2 more rapidly. A reaction pathway scheme consistent with experimental results and previous literature reports is proposed.