Crystals of Cs2NaYCl6 singly doped with Mo3+ or V3+ and a series of V3+:Mo3+ codoped Cs2NaYCl6 crystals were studied by low-temperature optical absorption and luminescence spectroscopy with particular focus on their near-infrared (NIR) to visible (vis) photon upconversion properties. In the codoped crystals 15 K Mo3+ T-2(2g) --> (4)A(2g) upconversion luminescence around 14300 cm(-1) is not only observed after direct excitation into the weak and narrow Mo3+ E-2(g) and T-2(1g) ground-state absorptions around 9700 cm(-1), but also after excitation into the broad V3+ T-3(1g) --> T-3(2g) absorption band between 10000 and 12500 cm-1. This is because the excitation energy is transferred from V3+ T-1(2g) to Mo3+ E-2(g) and this process is studied as a function of the relative V3+ and Mo3+ dopant concentrations. The Mo3+ 2T2g higher excited-state luminescence is not affected by the V3+ codopants, and this is because neither of the Mo3+ T-2(2g) emissions spectrally overlaps with V3+ absorption. The upconversion mechanisms which are active in the V3+:Mo3+ codoped Cs2NaYCl6 crystals at 15 K are identified on the basis of one- and two-color upconversion luminescence excitation spectroscopy. V3+ is found not only to act as a sensitizer for Mo3+ E-2(g), but also to participate in a nonradiative energy transfer upconversion (ETU) process, At 15 K, there is a strong spectral overlap of V-3 T-+ 3(1g) --> T-3(2g) ground-state absorption (GSA) with Mo3+ E-2(g) --> T-4(2g) excited-state absorption (ESA), leading to efficient GSA/ESA upconversion in 0.8% V3+:0.3% Mo3+ :Cs2NaYCl6 at 15 K. Excitation with the broadband NIR output of a tungsten lamp induces more than an order of magnitude more 15 K vis upconversion luminescence in 0.8% V3+ :0.3% Mo3+ :Cs2NaYCl6 than in purely 0.3% Mo3+ doped Cs2NaYCl6.