We observed the λ 4232 Å CH+ interstellar line in the direction of the Sco OB 1 association (d ≃ 2 kpc) with the aim of detecting variations of the 12C/13C ratio at large distances from the Sun. The collection of 30 high-resolution spectra (λ/δλ = 1O5) of the V-6.3 stars HD 152235 and HD 152424 by means of the ESO 3.6m telescope allowed us to detect in both lines of sight a weak 13CH+ absorption (Wλ ≃ 0.2-0.3 mÅ) and to measure the 12CH+/13CH+ abundance ratio, RCH+. We found RCH+ = 126±29 for HD 152235 and RCH+ = 98±19 for HD 152424. These ratios are in excess of the value RCH+ ≃ 65 measured in nearby interstellar clouds (d ≲ 0.5 kpc). An increase of the isotopic ratio in the general direction of the Galactic center is unexpected in the framework of Galactic chemical evolution models and our results indicate the presence of chemical inhomogeneities on distance scales of ≃ 1 kpc, or even shorter.