The crystal structures of the isostructural compounds Rb 2NaAl 3F 12 and K 2NaAl 3F 12 are solved by X-ray diffraction. The model was first established for the Rubidium compound in spite of systematic twinning, it was then confirmed by a Rietveld refinement of powder data and finally by refinement of data of untwinned crystal of K 2NaAl 3F 12. The symmetry is monoclinic: P2 1/m, Z=2, a=12.046(6) A ̊ , b=6.984(4) A ̊ , c=7.093(4) A ̊ and β=125.04(4)° for Rb 2NaAl 3F 12 (R=0.038 and R w=0.037) and a=11.882(7) A ̊ , b=6.983(4) A ̊ , c=6.942(4) A ̊ and β=125.59(3)° for K 2NaAl 3F 12 (R=0.025 and R w=0.027). The network, built up from (AlF 4 − H.T.B.-like layers, presents in fact a monoclinic distortion of the Cs 2NaAl 3F 12 structural type.