[en] The matrix isolation of ClO in a CO₂ matrix is investigated with the aim of providing a detection method for ClO in the stratosphere. Of main concern is the identification and interpretation of the so far unkown ESR-spectrum of matrixisolated ClO. After trapping of ClO in a CO₂ matrix at 10 K a highly anisotropic ESR-spectrum is observed which can be attributed to ClO. The shape and the g-values of the spectrum are characteristic for a ²PIsub(3/2) groundstate. Some minor structure discernible on the spectrum can be related to ClO at different trapping sites. After annealing the sample at T > 45 K, the radical concentration diminishes and the gsub(perpendicular to) peak splits into four peaks which correspond to the hyperfinesplitting of ClO. The values of the g and A tensors are determined by comparing the experimental spectrum with computer simulated spectra. In a simple crystal field model these values are compared with molecular parameters of ClO deduced from gas phase spectra. (orig.)