New fundamental K-O stretching frequencies of the isolated potassium peroxide K2O2 molecule are reported for different isotopic species. The B-2u vibration can be measured for (K2O2)-K-39-O-16, (K2OO)-K-39-O-16-O-18, (K2OO)-O-17-O-18, (K2O2)-K-39-O-18, (KKO2)-K-39-K-41-O-16 and (KKO2)-K-39-K-41-O-18. The B-3u vibration can be located for (K2O2)-O-16, (K2OO)-O-16-O-18 and (K2O2)-O-18 products, and the formerly B-3g symmetry motion, IR-activated by symmetry breaking in (K2OO)-K-39-O-16-O-18, (KKOO)-K-39-K-41-O-16-O-18, (KKO2)-K-39-K-41-O-16, and (KKO2)-K-39-K-41-O-18, is equally observed. The self-consistency of the attributions is based on semiempirical quadratic force field calculations, and, more importantly, upon theoretical ab initio and Density Functional Theory calculations. The ground state geometry calculated using the all electron density functional theory method corresponds to a slightly out of plane distorted rhombic structure (C-2v symmetry), in contrast to the D-2h Symmetry structure calculated for Li2O2 or at the SCF level for K2O2. The energy difference between rhombic D-2h and out-of-plane C-2v structure is nevertheless too small (0.08 kcal/mol) to exclude a calculation artefact. The agreement between the positions of observed vibrations and the density functional theory calculation predictions is however remarkable.