Significant effects on annealing on the signatures of the order-disorder transition in quiescent block copolymers were observed. We find that the disordering transition in cylindrically ordered block copolymers is accompanied by discontinuous changes in the small-angle X-ray scattering profiles, due to broadening of the structure factor. The sharpness of the observed transition is strongly affected by the size of the coherently ordered regions (grains), which, in turn, is determined by annealing history. The temperature of the transition was confirmed by optical birefringence measurements, which show a discontinuous decrease to zero at the same temperature.