We investigated the energy transfer dynamics in a phosphorescent host-guest system. Two spirobifluorene derivatives, [2,7-bis(2,2-diphenylvinyl)-9,9 ′-spirobifluorene] (DPVSBF) and [2,7-bis(1,2,2-triphenylvinyl)- 9,9′-spirobifluorene] (TPVSBF), were doped with the red phosphor complex [Os(bpftz)2(PPh2Me)2] (Os-R). The two hosts exhibit similar host-to-guest energy transfer efficiencies; however, the backward energy transfer from the Os-R complex to TPVSBF is slower than that from Os-R to DPVSBF. Quantum chemical calculations suggest that the backward energy transfer dynamics are related to the host's triplet-state energy level. The greater energy of the TPVSBF triplet state compared to that of DPVSBF plays a crucial role in its ability to confine triplet excitons in Os-R.