The structure of Gd3Ni5Al19 has been determined from single-crystal diffraction data [λ(MoKα) = 0.71073 Å, μ = 16.614 mm−1, F(000) = 2316, T = 293 K, wR = 0.023 for 997 contributing unique reflections]. This ternary aluminide crystallizes with a new, orthorhombic structure type, Pearson code oS108, (63) Cmcm, Wyckoff sequence f12c3, a = 4.0893(7), b = 15.993(2), c = 27.092(4) Å, V = 1771.8(5) Å3, Z = 4, Mr = 1277.95, Dx = 4.790 mg mm−3. The unit cell parameters of GdNiAl4 with orthorhombic YNiAl4 type, Pearson code oS24, (63) Cmcm, a = 4.0863(7), b = 15.520(4), c = 6.612(1)Å, were refined from single-crystal diffraction data [λ(MoKα) = 0.71073 Å, T = 293K]. The Gd3Ni5Al19 structure can be considered as an intergrowth of two kinds of slab, one cut from the orthorhombic YNiAl4 type and the second one corresponding to the translation unit of a hypothetical monoclinic R2T4Al15 structure. The general formula of this inhomogeneous linear structure series can be expressed as R2+mT4+mAl15+4m, where m is the number of consecutive intergrown YNiAl4-type slabs. The Gd3Ni5Al19 type is the simplest member of this structure series with m = 1. The following member with m = 2 is the monoclinic Y4Ni6Al23 type.