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The purpose of this study was to continuously monitor the pseudopolymorphic transition from anhydrate to monohydrate by measuring the NMR relaxation using time-domain NMR (TD-NMR). Taking advantage of the simplicity of the low-field NMR instrument configuration, which is an advantage of TD-NMR, the NMR instrument was connected to a humidity controller to monitor the pseudopolymorphic transition. First, ezetimibe (EZT) monohydrate was prepared from its anhydrate using a saturated salt solution method, and T 1 relaxation of EZT monohydrate and anhydrate was measured without a humidity controller. The T 1 relaxation results confirmed that EZT anhydrate and monohydrate could be distinguished using T 1 relaxation measurement. Next, continuous monitoring was conducted by TD-NMR and connected to a humidity controller. Anhydrous EZT was placed in an NMR glass tube and the T 1 relaxation measurement was repeated while maintaining the humidity on the side entering the NMR tube at 80% relative humidity. The T 1 relaxation became gradually faster from the initial to middle monitoring phases. The final T 1 relaxation was then recovered fully and these T 1 relaxation times were the same as the T 1 relaxation of EZT monohydrate. This study successfully monitored the pseudopolymorphic transition from EZT anhydrate to monohydrate via NMR relaxation.