Superhydrophobic alkyl ketene dimer (AKD) layers were successfully produced on top of untreated paper surfaces by a rapid expansion of supercritical CO2 solution (RESS) process. The new method resulted in a degree of hydrophobicity, as measured by contact angles of water droplets on AKD surfaces, dramatically higher, up to 173 degrees, compared to a conventional method consisting in melting AKD granules directly on the paper substrate, giving contact angles of around 109 degrees. Experiments were conducted to investigate the effects of varying pre-expansion pressure (100-300 bar), pre-expansion temperature (40 and 60 degrees C) and spraying distance (10 and 50 mm) on the properties of the treated surfaces. The surfaces were analyzed regarding AKD particle size, surface morphology and hydrophobicity with the aid of scanning electron microscopy (SEM) and contact angle measurements. The average AKD particle size after RESS processing was between 1 and 2 mu m depending upon the experimental conditions used, being slightly smaller when using higher pre-expansion pressure and temperature as well as shorter spraying distance. (C) 2008 Elsevier B.V. All rights reserved.