This study presents the structural characterization of the surface of four commercial nanofiltration membranes: NF90 (polyamide) and NF (polypiperazine amide) from Filmtec(TM) and NP010 and NP030 (polysulfone) from Microdyn Nadir(A (R)), by Atomic Force Microscopy (AFM). These membranes have been studied before and after undergoing a filtration process with potassium clavulanate. The fast Fourier filtering of AFM images with very high magnification (40 x 40 nm) has allowed identifying the pore size distribution and geometry of the pores on the surface of the membrane before their use. Images between 0.5 x 0.5 and 10 x 10 mu m(2) have allowed the study of the surface roughness of the samples before and after being used to filtrate potassium clavulanate solutions. The results of roughness and power spectral fractal dimension along with the skewness and kurtosis of the height distribution have been analyzed in terms of pore size, hydraulic permeability, and the adsorption of clavulanate for the different samples.