New quaternary intermetallic phases REMGa3Ge (1) (RE = Y, Sm, Tb, Gd, Er, Tm; M = Ni, Co) and RE3Ni3Ga8Ge3 (2) (RE = Sm, Gd) were obtained from exploratory reactions involving rare-earth elements (RE), transition metal (M), Ge, and excess liquid Ga the reactive solvent. The crystal structures were solved with single-crystal X-ray and electron diffraction. The crystals of 1 and 2 are tetragonal. Single-crystal X-ray data: YNiGa3Ge, a = 4.1748(10) Angstrom, c = 23.710(8) Angstrom, V = 413.24(2) Angstrom(3), 14/mmm, Z = 4; Gd3Ni3Ga8Ge3, a = 4.1809(18) Angstrom, c = 17.035(11) Angstrom, V = 297.8(3) Angstrom(,)(3) P4/mmm, Z = 1. Both compounds feature square nets of Ga atoms. The distribution of Ga and Ge atoms in the REMGa3Ge was determined with neutron diffraction. The neutron experiments revealed that in 1 the Ge atoms are specifically located at the 4e crystallographic site, while Ga atoms are at 4d and 8g. The crystal structures of these compounds are related and could, be derived from the consecutive stacking of disordered [MGa](2) puckered layers, monatomic RE-Ge planes and [MGa4Ge2] slabs. Complex superstructures with modulations occurring in the ab-plane and believed to be associated with the square nets of Ga atoms were found by electron diffraction. The magnetic measurements show antiferromagnetic ordering of the moments located on the RE atoms at low temperature, and Curie-Weiss behavior at higher temperatures with the values of mu(eff) close to those expected for RE3+ free ions.