In this work, cesium partially exchanged Keggin heteropolyacid (HPA) salts were prepared, characterized, and evaluated as solid catalysts in monoterpenes etherification reactions with alkyl alcohols. A comparison of the activity of soluble HPAs and their insoluble cesium salts showed that among three different Keggin anions the phosphotungstate was the most efficient catalyst. Assessments on the effects of the level of the protons exchange by cesium cations demonstrated that Cs2.5H0.5PW12O40 solid salt was the most active and selective phosphotungstate catalyst, converting β-pinene to α-terpinyl methyl ether. The influences of the main reaction parameters such as reaction temperature, time, catalyst load, substrate nature (i.e., alcohols and monoterpenes) were investigated. We have demonstrated that the simultaneous presence of the cesium ions and protons in the catalyst plays an essential role, being the 2.5–0.5 the optimum molar ratio. The Cs2.5H0.5PW12O40 salt was efficiently recovered and reused without loss of catalytic activity.