Interstellar CO 2 is an important reservoir of carbon and oxygen, and one of the major constituents of the icy mantles of dust grains, but it is not observable directly in the cold gas because has no permanent dipole moment. Its protonated form, HOCO + , is believed to be a good proxy for gaseous CO 2 . However, it has been detected in only a few star-forming regions so far, so that its interstellar chemistry is not well understood. We present new detections of HOCO + lines in 11 high-mass star-forming clumps. Our observations increase by more than three times the number of detections in starforming regions so far. We have derived beam-averaged abundances relative to H 2 in between 0.3 and 3.8 × 10 −11 . We have compared these values with the abundances of H 13 CO + , a possible gas-phase precursor of HOCO + , and CH 3 OH, a product of surface chemistry. We have found a positive correlation with H 13 CO + , while with CH 3 OH there is no correlation. We suggest that the gas-phase formation route starting from HCO + plays an important role in the formation of HOCO + , perhaps more relevant than protonation of CO 2 (upon evaporation of this latter from icy dust mantles).