Circumstellar envelopes (CSEs) around evolved stars are extraordinarily efficient factories of complex molecules and dust particles that will eventually enrich the ISM. To date, most chemistry studies have focused on C-rich CSEs, expected to show the largest molecular variety and abundances. We will present preliminary results from our ongoing millimeter line survey with the EMIR receivers of the IRAM 30m radiotelescope in two well known O-rich shells, IK Tau and OH231.8+4.2. In both targets, we detect hundreds of lines (many of them are first time detections in these objects) indicating that these stars are as chemically diverse as their Carbon counterparts. Some of the species preliminarily identified are: CO, SiO, H_2O, NH_3, NS, HCO^+, SO, SO_2, SH_2, OCS, HCN, HNC, CN, HC_3N, CS, H_2CO, HNCO, HNCS, SiS, N_2H^+, and a number of isotopologues (bearing 13C, 33S, 34S, 17O, 18O, 28Si, 29Si, 30Si, and 15N atoms), as well as complex organic molecules. Some of the intense transitions of C-, N-, and S-bearing species (including complex organic molecules) detected in OH 231.8 are absent in the spectrum of IK Tau. In contrast, molecules such as NaCl, NaCN, PO, and PN, are observed in IK Tau but not in OH 231.8. The origin of these remarkable differences is likely due to the different characteristics in the shocks in both objects.< Leer menos