Tungsten trioxide (WO₃) undergoes an electrochromic (EC) color change upon ion exchange via electrochemical redox reactions. Thin films of WO₃ are often fabricated by physical processes based on magnetron sputtering, and lithium is typically used as the cationic species that causes the color change. In a preliminary study, we achieved a suitable EC performance by using potassium as the cation in wet-processed films manufactured by a spin-coating method using WO₃ nanoparticles. However, comparisons of the EC properties of the WO₃ thin films prepared by various techniques are insufficient, and the behavior of potassium in these systems remains unclear. In this study, we observed that it was difficult for potassium to be swept smoothly into films prepared by sputtering or ion plating. In contrast, a wet-processed thin film obtained by spin coating was found to exhibit a good EC performance. This appeared to be largely attributable to structural variations arising from the film preparation methods.