The hydrogenation behavior of the Zintl phases MGa2 (M = Sr, Eu, Yb) was investigated. Upon being heated under 50 bar of hydrogen gas pressure, SrGa2 starts forming the polyanionic hydride SrGa2H2 at 125 °C as observed by in situ differential scanning calorimetry (DSC) and ex situ X-ray powder diffraction. The deuteration of SrGa2 was studied by in situ neutron powder diffraction at temperatures up to 300 °C in the pressure range of 50 ≤ p(D2) ≤ 61 bar with a time resolution of two minutes. Neither incorporation of deuterium into the interstitials of the crystal structure of SrGa2 nor any other intermediate of the reaction was observed. No significant variation in positional or occupational parameters as refined by the Rietveld method was observed for the deuteride SrGa2D2 under various temperature and pressure conditions (e.g., at T = 299 °C, p(D2) = 60 bar, space group P3m1, a = 4.4098(2) Å, c = 4.7429(3) Å, Sr in 1a, 0 0 0; Ga in 2d, 1/3 2/3 0.474(1); D in 2d, 1/3 2/3 0.120(2)). In situ DSC complemented by ex situ X-ray powder diffraction revealed that EuGa2 and YbGa2 do not form polyanionic hydrides in the investigated temperature-pressure ranges (20 °C ≤ T ≤ 220 °C, 5 bar ≤ p ≤ 54 bar). Instead binary hydrides and gallium-rich intermetallics are formed. The first refined crystal structural data are presented for YbGa6 in the tetragonal PuGa6-type structure (space group P4/nbm (No. 125), a = 6.0890(3) Å, c = 7.6562(4) Å, Yb in 2c, 3/4 1/4 0; Ga1 in 8m, 0.4750(4) 0.5250(4) 0.3377(4); Ga2 in 4g, 1/4 1/4 0.1691(5)).