Spectra of the ground-state transitions of HCN, HCO⁺, ¹²CO, and ¹³CO at the positions of seven GMCs along the major axis of M 33 (cf. Figs. 1, 2). ¹²CO and ¹³CO have been observed in position switching; HCN and HCO⁺ with wobbler switching. The spectra are shown on a main beam brightness temperature scale (T_mb). The velocity resolution is given by the spectrometer with the lowest resolution, i.e. WILMA, and is 5.4 km s^-1 in case of ¹²CO and ¹³CO, and 6.7 km s^-1 for HCN and HCO⁺. Center velocities are listed in Table 2. The local standard of rest (lsr) velocity displayed covers 300 km s^-1. The same velocity range is used to determine the baseline (red lines), excluding the line windows determined from ¹²CO  1–0 (dotted lines), cf. Fig. A.1.