We have studied thermal composition fluctuations of a ternary symmetric homopolymer/diblock copolymer system of PEE/PDMS/PEE-PDMS [PEE and PDMS being poly(ethyl ethylene) and poly(dimethyl siloxane), respectively] in its disordered state with small angle neutron scattering for concentration Phi of diblocks up to 15%. The phase diagram shows three characteristic regimes; (1) below the Lifshitz concentration Phi(LL)congruent to 9%; (2) in the very near vicinity of the Lifshitz concentration; and (3) above Phi(LL). In the regime (1) of low diblock content the maximum neutron intensity is obtained at Q = 0 and phase separation into macroscopic large domains is observed at low temperatures. With increasing diblock content the thermal fluctuations indicate a crossover from 3d-Ising to isotropic Lifshitz critical behavior with critical exponents of the susceptibility gamma = (1.62 +/- 0.01) and correlation length nu = (0.99 +/- 0.04) appreciably larger than in the 3d-Ising case. In the structure factor this crossover is accompanied by a strong reduction of the Q(2) term leading to the dominance of the Q(4) term; the restoring force of the thermal fluctuations is strongly reduced as the Q(2) term is proportional to the surface energy. Near the Lifshitz critical temperature a further crossover was observed leading to the appreciably larger critical exponents gamma = (2.44 +/- 0.08) and nu = (1.22 +/- 0.08) and a stabilization of the disordered regime visible through a decrease of the phase boundary by nearly 10 K. This crossover is interpreted by the formation of fluctuation induced inhomogeneous diblock distribution at the interface of the thermal fluctuations. (2) In the intermediate regime between 9% and 12% diblock content the Lifshitz line was crossed twice upon increasing the temperature from low to high temperatures; at low and high temperatures the structure factor S(Q) shows diblock character (maximum of S(Q) at Q not equal 0) while at intermediate temperature blendlike character (maximum of S(Q) at Q = 0). At low temperatures a transition to a bicontinuous microemulsion phase is proposed. (3) At diblock content of 15% a weak order-disorder transition was observed. The data in the Lifshitz critical range and larger than the Lifshitz line could be interpreted by a recently developed theory of Kielhorn and Muthukumar who considered the effect of thermal fluctuations in ternary homopolymer/diblock copolymer samples and from which the Flory-Huggins parameter could be evaluated. (C) 2000 American Institute of Physics. [S0021-9606(00)50512-1].