The adsorption of polyacrylate ammonium dispersant on alumina particles was analyzed for alumina slurries with various solid loadings up to 57.5vol% by FT-IR spectroscopy, with which the rheological properties were correlated. For alumina slurries of pH around 9.3, unadsorbed dispersant molecules always existed in the dispersion media even when the amount of the dispersant was insufficient for optimum dispersion. When solid loading was higher than a critical value (50vol%, in the present case), the adsorption of dispersant was suppressed with increasing solid loading. Correspondingly, the agglomeration of the slurries became pronounced. At the critical solid loading, the mean interparticle distance was estimated to be close to the length of the dispersant molecule. It was assumed that, in such highly concentrated slurries, the adsorption of the dispersant molecules should be suppressed.