The indirect laser-induced temperature jump (ILIT) method is used to determine the acidity (pK(a)) of monolayers composed of 11-mercaptoundecanoic acid (MUA) self-assembled on vapor-deposited gold film electrodes in contact with either 0.10 or 1.0 M ionic strength NaClO4 electrolyte solutions. The ILIT technique may be used to measure the pK(a) of a surface-attached acid because the magnitude of the ILIT response is related to the potential gradient at the electrode/electrolyte interface which is, in turn, related to the extent of ionization of the acid. The relevant data are the potentials of zero ILIT response versus pH. A simple two-layer model of the acid-modified electrode/electrolyte interface and Gouy-Chapman double layer theory (modified for the onset of dielectric saturation effected by the large concentration of charge present at this interface when a substantial fraction of the carboxylic acid moieties are ionized) were employed to analyze these data. This analysis gives pK(a) = 5.7 +/- 0.2 at 0.10 M ionic strength and pK(a) = 4.4 +/- 0.2 at 1.0 M ionic strength while the total concentration of MUA comprising the self-assembled monolayer is determined to be essentially the same at both ionic strengths, averaging (5.4 +/- 0.3) x 10(-10) mol/cm(2). Reasons for the observed variation of pK(a) with ionic strength are discussed.