Citric acid etchants with ferric chloride (10-3) promote adhesion and are effective etchants. We used atomic force microscopy (AFM) to determine the dentin etching characteristics of 10% citric acid etchant with various additions of ferric chloride. AFM was used to measure the surface recession and morphology of dentin etching for a variety of 10% citric + Fe-Cl etchants. Commercial 10-3 was used as the control. Dentin disks were prepared and partially masked during etching so that the unetched dentin served as a reference. AFM images of the same samples were taken at each of the seven experimental steps using 10% citric acid with various quantities of ferric chloride ranging from 0 to 3% (denoted 10-0, 10-1, 10-1.8 and 10-3, respectively) which altered the pH from 1.63 to 0.42. Changes in depth (nm) relative to the reference height for intertubular dentin were determined after etching 15 s; clinical air drying, rewet for 1-5 min; desiccation for 24 h; rehydration for 24 h. Differences of log(depth change) were tested with mixed effects cell mean models. Overall differences were significant (P < 0.001). Significant differences were found between etchants and the extent of collapse on drying was dependent on pH and Fe-Cl content. Following long-term rehydration, samples recovered, but those with 0 or 1% Fe-Cl did not recover as completely (P < 0.01). Etching and collapse depended on Fe-Cl content. The collapse was nearly reversed on rehydration. Ten per cent citric acid with ferric chloride did not prevent the collapse of etched dentin. Ferric chloride additions reduced pH and increased the collapse of dentin on drying for either brief or prolonged periods. Brief rewetting can reverse the effect of brief air drying. For fully desiccated samples the effect was not reversed after 30 s, but was largely or completely reversed at 24 h in solution depending on Fe-Cl content.