Nucleation is a process consisting of a succession of unions of small gas-phase molecules leading to the formation and growth of droplets and particles. Consequently, it is a crucial process in the formation of clouds and dust particles in a range of systems including planetary atmospheres and circumstellar shells. Under certain conditions, low temperatures and/or high degrees of supersaturation, nucleation can become a barrierless process, and dimerization, the complexation of the two smallest molecular species involved, becomes the rate-limiting step of nucleation. Previous investigations of the kinetics of formation of homogeneous dimers using the CRESU (a French acronym standing for reaction kinetics in uniform supersonic flow) technique have mainly used mass spectrometric detection. However, in this research 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 a heterogeneous dimer. 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 formic acid – CO2 heterodimers at temperatures down to 35 K or below. Both molecules are present in a wide range of interstellar ices, dense molecular clouds, star-forming regions, cometary comae and planetary atmospheres, where the formation of the heterodimer may be key in the nucleation and condensation processes taking place at these environments.