The 15 K visible (VIS) upconversion-luminescence of Cs2ZrBr6:0.2% Os4+ upon excitation in the near-infrared (NIR) is presented and analyzed. Crystals of Cs2ZrBr6:0.2% Os4+ Were synthesized and grown by the Bridgman technique. At 15 K VIS luminescence around 16000 cm(-1) is induced by NIR excitation around 11400 and 13400 cm(-1). Due to the low maximum vibrational energy of 220 cm(-1). four excited states participate in the upconversion processes. A photon avalanche and a ground-state absorption/excited-state absorption sequence are identified as upconversion mechanisms by high-resolution upconversion-excitation spectra and by power-dependent and time-resolved measurements. The lifetimes of the excited states Gamma(4) ((3) T-1g), Gamma(5) ((3) T (1g)), Gamma(5) ((1) T-2g), and Gamma(1) ((1) A(1g)) at 15 K are 45 ms, 1.6 ms, 261 mu s, and 8 mu s, respectively. The dynamics of the upconversion are simulated with a rate-equation model.