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Polyoxometalates (POMs) are a large group of metal oxide clusters with well-defined structures, diverse properties, and various applications. Since solutions of metal aquo complexes can protonate their coordinated water ligands, some POMs can behave like weak Brønsted nanoacids. Playing an important role not only in the solution behaviors but also in applications as catalysts and biologically active materials, the acidity of POMs is sometimes related to their deprotonation behaviors. Therefore, studies on the deprotonation of POMs can help people understand the nature of POMs, especially the acidity feature, to further explore POM-based materials and devices. In this work, a series of lanthanide clusters are applied as model systems to understand the deprotonation of the clusters, which is related to the relative strength of coordinated water molecules and determines the acidity feature of the POM cluster. With potassium ions as counterions, these lanthanide clusters are macroanions in aqueous solutions. The aqueous solutions of the POMs have certain pH values and conductivity, and there is no proton released or absorbed. But the addition of base can trigger deprotonation from the lanthanide clusters. It was found that these lanthanide clusters demonstrated different deprotonation capacity, and according different acidity features. This acidity variation might derive from the difference among lanthanide ions caused by lanthanide contraction.