Catalyst composition for the oligomerization of ethylene, comprising (i) an at least partially hydrolyzed transition metal compound, obtainable by controlled addition of water to a transition metal compound having the general formula MXm(OR′)4-m or MXm(OOCR′)4-m, wherein R′ is an alkyl, alkenyl, aryl, aralkyl or cycloalkyl group, X is halogen, preferably Cl or Br, and m is from 0 to 4; preferably 0-3; and (ii) an organoaluminum compound as a cocatalyst, wherein the molar ratio of water and transition metal compound is within a range of between about (0.01-3):1; a process for oligomerization of ethylene and a method for preparing the catalyst composition.