The infrared spectrum of OH in Ar matrices at temperatures of 11.5 +/- 0.5 K has been investigated as a function of water concentration for H2O:Ar ratios ranging from 1:70 to 1:1000. These data and evidence gathered from earlier experimental and theoretical work lead to an assignment of bands at 3452 and 3428 cm(-1) to the H2O . HO complex. This is the first time that a definitive assignment has been made to an isolated cm H2O . HO complex. This observation may have important implications for fields as diverse as molecular biology, astrochemistry, atmospheric chemistry, and electrochemistry.