The rotational spectrum of the 1:1 complex formic acid-isopropylformate (FA-IPF) has been first observed when trying to assign the pulsed jet Fourier transform microwave (FTMW) spectrum of the adduct formic acid-2-propanol, by expanding a binary mixture of HCOOH and 2-propanol in He. The strong FTMW spectrum of isopropylformate, formed by the esterification reaction, was observed instead. However, when HCOOH was in excess in the binary mixture, it was possible to observe and assign the rotational spectrum of FA-IPF. Later on a much intense spectrum of FA-IPF was obtained, when combining FA with IPF. Finally, the spectra of five isotopologues of the most stable isomer of formic acid-isopropylformate have been observed by means of rotational spectroscopy in supersonic expansion. Some of them, HCOOH-(CH3)(2)CHOOCD and HCOOH-(CH3)(2)CDOOCH have been synthesized in the MW cavity by using DCOOH or (CH3)(2)CDOH as precursors in the esterification process. In the observed isomer of the complex, the two subunits are linked to each other by a standard O-H...O and a weak C-H...O hydrogen bond. The dissociation energy has been estimated to be 34.1 kJ.mol(-1).