The ensemble smoother with multiple data assimilation (ES-MDA) coupled to a normal-score transformation is used to fit a Langmuir isotherm curve to estimate its parameters (Sm and b) and their uncertainty. The highlights of this work are three: i) the ES-MDA can be used as a curve fitting procedure, ii) the ES-MDA provides also a full uncertainty quantification about the fitted parameters and iii) for the specific case of the Langmuir isotherm, parameter Sm is well identified with little uncertainty, while parameter b is well identified with a larger uncertainty, indicative that solute concentrations are more sensitive to Sm than to b. As a by-product, the number of samples required to characterize the joint uncertainty of Langmuir isotherm parameters is also investigated; it can be concluded that the minimum number of samples to use is six, with best results