Two specific problems are identified in understanding the data on the Quaternary of Montane areas in the Sunda-Sahul region. These are: (1) The Upper Montane rainforest, according to palynological evidence, appears to have been absent in the Late Pleistocene, although it is well represented today. (2) The estimates of temperature lowering in the Late Pleistocene are strikingly greater in the mountains than in the lowlands. A single possible explanation for both problems is proposed. It is argued that altitudinal zonation of vegetation is partly controlled by ultraviolet-B light which is strikingly increased at high altitudes in the tropics. This fact could account for both the above problems, not only in the Sunda-Sahul region, but also in other tropical regions.