Analysis of the SiO line emission in the IRAS 18264−1152 a) and IRAS 18151−1208_2 b) outflows observed with the APEX (J = 8–7, present work) and IRAM 30 m (J = 3–2, from López-Sepulcre et al. 2011) antennas. Both lines have been observed with a well-matching beam (~18″): the solutions for the observed SiO(8−7)/SiO(3−2) intensity ratio are shown in the n_H2–T_kin-plane for non-LTE (RADEX) plane-parallel models for both the optically thin (N(SiO) = 10¹¹ cm^-2) and optically thick (N(SiO) = 10¹⁶ cm^-2) cases. Contours are for the measured ratios at the typical red- (+54 km s^-1, for IRAS 18264−1152; +50 km s^-1, for IRAS 18151−1208_2) and blue-shifted outflow velocities (+15 km s^-1 for IRAS 18151−1208_2), as well as at the systemic velocity (IRAS 18264−1152: +43.9 km s^-1, IRAS 18151−1208_2: +30 km s^-1). Dashed contours are for uncertainties.