MnV 2 O 6 (below 540°C) and AgVMoO 6 are monoclinic C2/m brannerite-type structures. They show complete miscibility, forming MnAg ؍ Mn 1؊y Ag y V 2؊y Mo 1؉y O 6 solid solutions. MnAg is able to incorporate an excess of MoO 3 and form MnAg ؍ Mn 1؊x؊y x Ag y V 2؊2x؊y Mo 2x؉y O 6 phases ( ؍ cation vacancy in the original Mn/Ag site) belonging to the pseudoternary MnV 2 O 6 -AgVMoO 6 -MoO 3 system. The system can be visualized as an equilateral triangle with X ؍ 100x and Y ؍ 100y coordinates along the MnV 2 O 6 -MoO 3 and MnV 2 O 6 -AgVMoO 6 arms, respectively. The limit of the existence of MnAg is almost linear and extends between the X-Y points S 1 ؍ (45, 00) and S 2 ؍ (12, 88). As the ionic radii of dopants (Ag ؉ /Mn 2؉ , Mo 6؉ /V 5؉ ) are larger than those of the mother cations, the lattice constants in both series-MnAg and almost saturated MnAg (located along the S 1 S 2 curve-rise systematically with an increase in Y. Behind the S 1 S 2 curve, saturated MnAg has been observed accompanied by ''hexagonal MoO 3 '' (h-MoO 3 ), Y phase ؍ Mn 0•15 V 0.3 Mo 0.7 O 6 , by (Mo x V 1؊x ) 2 O 5 , and sometimes by orthorhombic MoO 3 and MnMoO 4 . According to the literature, so-called h-MoO 3 is a defective tunnel structure stabilized by the inserted free or hydrated L ؍ K ؉ , Rb ؉ , Cs ؉ , NH ؉ 4 ions. The formula of dry h-MoO 3 is L 0.13 V 0.13 Mo 0.87 O 3 (I). Ag was believed to be too small to stabilize the peculiar h-MnO 3 . Despite that, we observed Ag-containing h-MoO 3 in the field of the MnV 2 O 6 -AgVMoO 6 -MoO 3 triangle. Attempts to obtain a pure Ag-containing h-MoO 3 have failed so far; apparently the presence of Mn (beside Ag) stabilizes the h-MoO 3 structure and the formula is more complex as compared with I. The phase diagram of the MnV 2 O 6 -AgVMoO 6 system reveals great similarity to those containing Li and Na instead of Ag. All three diagrams involve a narrow lens-type solidus-liquidus gap at high Y and two peritectic meltings at lower Y, yielding, besides liquid, the hightemperature -MnAg polymorph and Mn 2 V 2 O 7 .