Three methods were used to obtain values of the sensitivity constant (Cf) of an electrochemical quartz crystal microbalance (EQCM) using solutions of Tl+ ions: chronoamperometry, chronopotentiometry and cyclic voltammetry. The results obtained by these three methods showed that the Cf values were very sensitive to experimental conditions used to form deposits and the three techniques revealed that Cf is markedly influenced by the roughness of thallium films. For example, if the thickness is low or too high, the relative error of Cf with respect to the theoretical value is in the order of −20% due to its roughness and mechanical stress produced by the difference in atomic size. On the contrary, if the thicknesses are intermediate, the Cf values approach the theoretical value with errors that may be below +5%. Furthermore, the electrodes employed in this determination can be used again without important modifications in their surface composition.