Phase-resolved medium-resolution VLT spectroscopy of the low-mass X-ray binary GX9+9 has revealed narrow CIII emission lines that move in phase relative to our new estimate of the ephemeris, and show a velocity amplitude of 230 +/- 35 km s-1. We identify the origin of these lines as coming from the surface of the donor star, thereby providing the first estimate of the mass function of f(M1) >= 0.22Msolar. Rotational broadening estimates together with assumptions for the mass donor give 0.07 <= q <= 0.35 and 182 <= K2 <= 406 km s-1. Despite a low-mass ratio, there is no evidence for a superhump in our data set. Doppler maps of GX9+9 show the presence of a stream overflow, either in the form of material flowing downward along the accretion disc rim or in a similar fashion as occurs in high mass transfer rate cataclysmic variables known as the SWSex stars. Finally, we note that the Bowen region in GX9+9 is dominated by CIII instead of NIII emission as has been the case for most other X-ray binaries.