Relaxation calorimetry experiments were used to measure the temperature dependence of the heat capacity near the order-disorder phase transition (ODT) in a low molecular weight volumetrically symmetric, lamellae-forming, poly(1,4-isoprene-b-dl-lactide) diblock copolymer with Mn = 2750 g/mol (dispersity cross sign D = 1.10) and composition fPLA = 0.51. This enabled accurate determination of the latent heat of the ODT, yielding a value ΔHODT = 0.26 ± 0.02 J/g, consistent with previous measurements using differential scanning calorimetry. The relatively small magnitude of the latent heat supports the results of recent molecular simulations and reinforces the importance of composition fluctuations on the thermodynamics of block polymers near the ODT. These thermal measurements reveal no signature of the fluctuation effects in the disordered state as T → TODT, consistent with previous experimental and theoretical work, which supports the notion that composition fluctuations develop over a relatively wide range of temperatures for T > TODT. The finite width of the heat capacity peak, ΔTODT ≈ 1 °C, is shown to be consistent with molecular dispersity. These measurements demonstrate the utility of relaxation calorimetry as a quantitative semistatic thermal characterization tool for block polymers in the vicinity of phase transitions. (Graph Presented).