The Heterodyne Instrument for the Far- Infrared (HIFI) was launched aboard the Herschel space telescope on the 14th of May 2009. HIFI's frequency range is spread over 7 mixer bands. Bands 1-5 (480-1270 GHz) use Superconducting-Insulator- Superconducting (SIS) mixer technology while bands 6 & 7 (1410-1910 GHz) use Hot Electron Bolometer (HEB) mixer technology. HIFI is a double sideband instrument and hence contains both the upper and lower sideband of the down converted sky signal. The gain in the upper and lower sideband is not always equal. This effect introduces a calibration uncertainty that must be understood in order to achieve the HIFI calibration goal of 3%. To determine the frequency dependent sideband ratio for each mixer band, a gas cell test set up was developed . During the instrument level testing a number of simple (12CO, 13CO and OCS) and complex (CH3CN and CH3OH) molecules were observed using the HIFI instrument. Using a radiative transfer model with the measured pressure and optical path length of the gas cell and molecular line parameters taken from the JPL and HITRAN catalogs, model spectra can be generated. By comparing the generated spectra with the observed spectra the sideband gain can be determined. In this paper we present the analysis of 12CO gas cell data in bands 1 & 2 and the application of the determined side gain ratios to flight data.