Monte Carlo simulations have been performed with a lattice model that represents systems of symmetric diblock copolymer chains immersed in a nonselective solvent of varying thermodynamic quality. Different values for the polymer concentration and the energy parameter that gauges solvent quality have been set. This way, the chain dimensions and the solvent segregation and microphase separation transitions have been numerically characterized for two different chain lengths. The results are discussed and compared with existing theoretical predictions (mean-field and self-consistent-field treatments for Gaussian chains).