Out-of-plane chemically ordered transitionmetal boride(o-MAB) phases, Ta4M & DPRIME;SiB2 (M & DPRIME; = V, Cr), and a structurally equivalent disordered solidsolution MAB phase, Ta4MoSiB2, are synthesized.DFT calculations are used to examine the dynamic stability, elasticproperties, and electronic density states of the MAB phases. We report on the synthesis of computationally predictedout-of-planechemically ordered transition metal borides labeled o-MAB phases, Ta4M & DPRIME;SiB2 (M & DPRIME; =V, Cr), and a structurally equivalent disordered solid solution MABphase Ta4MoSiB2. The boride phases were preparedusing solid-state reaction sintering of the constituting elements.High-resolution scanning transmission electron microscopy along withRietveld refinement of the powder-X-ray diffraction patterns revealedthat the synthesized o-MAB phases Ta4CrSiB2 (98 wt % purity) and Ta4VSiB2 (81 wt% purity) possess chemical ordering with Ta preferentially residingin the 16l position and Cr and V in the 4c position, whereas Ta4MoSiB2 (46wt % purity) was concluded to form a disordered solid solution. Densityfunctional theory (DFT) calculations were used to investigate thedynamic stability, elastic properties, and electronic density statesfor the MAB phases, confirming the stability and suggesting the boridesbased on Cr and Mo to be stiffer than those based on V and Nb.