Control of bacterial adhesion is important in many industrial and medical applications. Ionic strength and pH are known to affect the process of attachment; however their influence on bacteria/surface distances has been inferred but not quantitatively established. The distances between attached bacteria and surfaces have only recently been measured. In this paper ATR-IR spectroscopy has been used to study the influence of pH and ionic strength on the distance of freshly attached Pseudomonas aeruginosa from ZnSe surfaces. Reversible changes in the absorbance of P. aeruginosa were observed for changes of pH between 4 and 10 at a constant ionic strength of 0.003 mol L-1 and for changes of ionic strength between 0 and 0.15 mol L-1 at a constant pH of 6.3. An increase in ionic strength from 0 to 0.003 mol L-1 led to an increase in amide II absorbance of 30%, which corresponds to an average movement of 120 nm toward the surface. A movement of this magnitude is likely to be due to changes in the length of bacterial surface polymers. The influence on bacterial surface polymer lengths of ionic strengths in this range has not been widely considered in models of bacterial attachment.