Directly compressed tablets manufactured with high concentrations of microcrystalline cellulose tend to soften when exposed to high humidity. Modification/co-processing of microcrystalline cellulose with hydrophobic agents using two methods was proposed to prevent hardness decrease of placebo tablets upon moisture absorption. Hardness of placebo tablets before and after exposure to humidity of 75% RH was compared. Hardness of all tested placebo tablets decreased after exposure to humidity of 75% RH, however the extent of decrease was different. Relative hardness decrease was the lowest for modification with use of hydrogenated castor oil. Co-processing of microcrystalline cellulose by 20% addition of hydrogenated castor oil allows to reduce a hardness decrease upon moisture uptake.