Monolayer arrays of polystyrene−poly(2-vinylpyridine) diblock copolymer cylinders with excellent orientational order and a very low density of dislocations are prepared by cooling slowly from above the bulk order−disorder temperature (ODT) ∼212 °C within silicon oxide channels one cylinder spacing a in depth and 2 μm in width. The translational order of this array, however, is short range with a correlation length of ∼12a. If such an array is heated to a temperature above the glass transition temperature of the block copolymer (100 °C) but well below the ODT, a finite density of thermally generated dislocations is observed, which leads to a decrease in the translational correlation length and an appearance of quasi-long-range orientational order such that the orientational correlation function g₂(r) decays as a power law, i.e., g₂(r) ≈ (r/a)^-η₂(T). The state of disorder at any given temperature appears to be an equilibrium one since cylinder arrays with similar dislocation densities and correlation functions can be obtained either by heating from the well-ordered state or by cooling slowly directly to the final temperature and holding at that temperature for a sufficient time. Above a temperature of 195 °C, the orientational order becomes short range (g₂(r) decaying exponentially with r), and a large density of disclinations is observed in addition to the dislocations. The cylinder array becomes isotropic above this temperature, which is ∼20 °C below the bulk ODT. These results are in qualitative agreement with the theory of Toner and Nelson which describes the thermal generation of disorder and ultimate melting of a two-dimensional smectic.