[en] Highlights: • Question use of inhibition efficiency to obtain standard Gibbs energy of adsorption. • Demonstrate that inhibition efficiency is not always equivalent to surface coverage. • Probe validity of applying the Langmuir isotherm to a corrosion inhibitor system. -- Abstract: The common practice of using inhibition efficiency (η%) to determine the standard Gibbs energy of adsorption (${\text{ΔG}}_{\text{ads}}^{\text{o}}$) for a corrosion inhibitor in acidic solution is evaluated. It is demonstrated that the typical assumption that η% is a good proxy for fractional surface coverage (θ) is not necessarily valid. Consequently, the accuracy of the ${\text{ΔG}}_{\text{ads}}^{\text{o}}$ value obtained from such data is doubtful. Moreover, it is argued that even a more direct measurement of θ may still not allow one to extract an accurate estimate of ${\text{ΔG}}_{\text{ads}}^{\text{o}}$.