The visible (VIS) upconversion luminescence of Cs2ZrCl6: x% Os4+ (x=0.1,1) upon excitation in the near infrared (NIR) is presented and analyzed. The crystals of Cs2ZrCl6: x% Os4+ (x=0.1,1) were synthesized by the Bridgman technique. VIS luminescence around 17 000 cm(-1) is induced by NIR excitation between 10 500 and 14 500 cm(-1). The three excited states Gamma (4) (T-3(1g)), Gamma (5) (T-1(2g)), and Gamma (1) ((1)A(1g)) with 15 K lifetimes of 20 ms, 8 mus, and 4 mus, respectively, in Cs2ZrCl6: 1% Os4+ participate in the upconversion. Excitation at the Gamma (4) (T-3(1g))--> Gamma (1) ((1)A(1g)) energy shows the typical fingerprints of a photon avalanche. The dynamics of this excitation are calculated with a rate-equation model, and the observed concentration and temperature dependence of the processes are analyzed. The upconversion behavior of Cs2ZrCl6: x% Os4+ (x=0.1,1) is compared with the bromide analog Cs2ZrBr6: 0.2% Os4+.