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This paper presents the physical properties of a nonstandard orthorhombic form of MnV 2 O 6 , including a comparison with the isostructural orthorhombic niobate MnNb 2 O 6 , and with the usual MnV 2 O 6 monoclinic polymorph. Orthorhombic (P bcn) MnV 2 O 6 is obtained under extreme conditions of high pressure (6.7 GPa) and high temperature (800 • C). A negative Curie-Weiss temperature θ CW is observed, implying dominant antiferromagnetic interactions at high temperatures, in contrast to the positive θ CW of the monoclinic form. Specific-heat measurements are reported down to 1.8 K for all three compounds, and corroborate the magnetic-transition temperatures obtained from susceptibility data. Orthorhombic MnV 2 O 6 presents a transition to an ordered antiferromagnetic state at T N = 4.7 K. Its magnetic structure, determined by neutron diffraction, is unique among the columbite compounds, being characterized by a commensurate propagation vector k = (0,0, 1 2). It presents antiferromagnetic chains running along the c axis, but with a different spin pattern in comparison to the chains observed in MnNb 2 O 6. By a comparative discussion of our observations in this three compounds, we are able to highlight the interplay between competing interactions and dimensionality that yield their magnetic properties.