The bond strength of chlorine peroxide (ClOOCl) is studied by photoionization mass spectrometry. The experimental results are,obtained from the fragmentation threshold yielding ClO+, which is observed at 11.52 +/- 0.025 eV. The O-O bond strength D-o is derived from this value in comparison to the first ionization energy of ClO, yielding D-298(o) = 72.39 +/- 2.8 kJ mol(-1). The present work provides a new and independent method to examine the equilibrium constant K-eq for chlorine peroxide formation via dimerization of ClO in the stratosphere. This yields an approximation for the equilibrium constant in the stratospheric temperature regime between 190 and 230 K of the form K-eq = 1.92 x 10(-27) cm(3) molecules(-1) x exp(8430 K/T). This value of K-eq is lower than current reference data and agrees well with high altitude aircraft measurements within their scattering range. Considering the error limits of the present experimental results and the resulting equilibrium constant, there is agreement with previous works, but the upper limit of current reference values appears to be too high. This result is discussed along with possible atmospheric implications.