Thermal rate constants of the complex-forming bimolecular reaction HO+CO double left right arrow HOCO-->H+CO2 were measured between 90 and 830 K in the bath gas He over the pressure range 1-700 bar. In addition, the vibrational relaxation of HO in collisions with CO was studied between 300 and 800 K. HO was generated by laser photolysis and monitored by saturated laser-induced fluorescence. The derived second-order rate coefficients showed a pronounced pressure and complicated non-Arrhenius temperature dependence. Above 650 K, the disappearance of HO followed a biexponential time law, indicating thermal instability of collisionally stabilized HOCO. By analyzing the corresponding results, an enthalpy of formation of HOCO of Delta H-f,0(o) = -(205+/-10) kJ mol(-1) was derived. On the basis of energy- and angular-momentum-dependent rates of HOCO formation, activated complex properties for the addition reaction HO+CO-->HOCO were derived from the limiting high-pressure rate constants; with the limiting low-pressure rate constants, activated complex properties for the dissociation HOCO-->HS+CO2 could be fitted as well. The observed transitions between low- and high-pressure limiting rate constants were well reproduced with these molecular parameters and collisional contributions; some evidence for rotational effects in collisional energy transfer was found. The surprisingly successful theoretical modeling of all available experimental data (80-2800 K, 0.0001-700 bar) allows for a satisfactory data representation of the rate coefficients over very wide ranges of conditions.