Nucleation, which is the first step from vapour to condensed phase, plays an important role in a wide range of environments such as atmospheric clouds, planetary atmospheres, comets or circumstellar shells. However, its onset is still poorly understood. Under the appropriate conditions, such as low temperature and/or high degrees of supersaturation, the rate-limiting process of nucleation is the formation of the first dimer by means of these intermolecular interactions. The kinetics of formation of homodimers have been investigated in a few previous studies, using the CRESU (reaction kinetics in uniform supersonic flow) technique mainly coupled with mass spectrometric detection. Here we have employed a completely new detection scheme, chirped-pulse Fourier transform mm-wave spectroscopy, to study for the first time the kinetics of formation of heterodimers. This innovative technique combines the ability to generate continuous cold uniform supersonic flows with the high selectivity and general applicability of rotational spectroscopy, allowing us to follow both reactant and product concentrations simultaneously. Furthermore, the high sensitivity achieved has allowed us to employ pseudo-first-order conditions to obtain absolute rate constants. We will report the first measurements of rate constants for the formation of a set of heterodimers, including formic acid, CO2 and water among others, at temperatures between 35 and 80 K.