1-Phenyl-3-pyrazolidone (PhP) having an active hydrogen was blocked by phenyl isocyanate derivatives to reduce its nucleophilicity, and the potential of the blocked PhP as a thermal latent reductant generating the organic reducing agent, PhP, was revealed for the first time. The degree of dissociation of the blocked PhP during heating in solution can be controlled by the steric hindrance of the isocyanate-blocking agents. The thermal latency of the blocked PhP for the sorbic ester-based polyperoxides, which undergo a serious reductive decomposition by PhP even at 30 °C, was investigated to accelerate the decomposition of the polyperoxides only at elevated temperatures. It was revealed that the stability of the polyperoxides was not significantly affected at 30 °C, and marked reductive decomposition of the polyperoxides together with thermal decomposition took place at elevated temperatures. For the block PhPs, which are highly compatible with the polyperoxides, the decomposition promotion effect increased with increasing the steric hindrance of the blocking agents, and the controlled decomposition of the polyperoxides was successfully achieved. Moreover, the addition of the blocked PhP resulted in the complete solubilization of sorbic ester-based cross-linked polyperoxides by heating, which was not able to be achieved without the blocked PhP. The performance of the debondable adhesive by using the sorbic ester-based cross-linked polyperoxides was improved by the addition of the blocked PhP, although hazardous isocyanate formation during the activation process may limit the practical application. Specifically, a faster decrease in the lap-shear adhesion strength by heating was accomplished. Furthermore, the same level of dismantlability was achieved in the wide range of heating temperatures and time, which is an important requirement for polyperoxides undergoing significant exothermic decomposition bringing about undesired temperature rise.