Abstract: Hydrolytic polymerization of chromium(Ⅲ)inM Cr(NO₃)₃ aqueous solution in presence of fumaric acid of four different concentrations(0.0035,0.005,0.0075,0.01M)was studied.The results show that the state of chromium(Ⅲ)is guite complicated,and besides the species([Cr₂(OH)A]³⁺,[Cr₂(OH)A₂]⁺,[Cr(OH)A])which exist in high and low concentration regions,new species([Cr₂(OH)₂A]²⁺has also been found,so this region may be regarded as transition stage,The species in the transition stage would transform one to another as the concentration changes.It shows some analogy with the results presented previously.Such phenomenon is rarely observed in hydrolytic reaction of other metal ions.Hydrolysis constants are calculated by program LEMITand values of identical species obtained in the different concentration regions of fumaric acid are in good agreement with each other and agree with those reported in the previous papers.