The ground-state rotational spectra of the formaldehydedimethyl ether (H2CO-DMS) and formaldehydedimethyl sulfide (H2CO-DMS) complexes have been studied by Fourier transform microwave spectroscopy. The a-type and c-type rotational transitions have been assigned for the normal and deutrated formaldehyde-containing species of both complexes. In the case of H2CO-DMS, doublets were observed with the splitting 10300 kHz, whereas no such splittings were observed for H2CO-DMS, D2CO-DME, and D2CO-DMS. The observed rotational spectra were found consistent with a structure of Cs symmetry with DME or DMS bound to H2CO by two types of hydrogen bonds: CH(DME/DMS)-O(H2CO) and O(DME)/S(DMS)-HC(H2CO). The Rcm distances between the centers of mass of the component molecules in the H2CO-DMS and H2CO-DMS complexes were determined to be 3.102 and 3.200 angstrom, respectively, which are shorter than those in most related complexes. The spectral and NBO analyses showed that H2CO-DMS has a stronger charge transfer interaction than H2CO-DMS does and that the binding energy of H2CO-DMS is larger than that of H2CO-DMS.