The yellow substance, gilvin, is an important component of water quality in natural waters. Quantitative study of gilvin production is difficult since gilvin is not a well defined substance. In the present study, leaf litter from four common tree species of south-western Australia were exposed to rainy weather for 183 days and leachate properties were monitored. The gilvin leaching from litter was estimated using a ‘humic acid (HA) equivalent’ concept, derived by drawing an analogy between the spectrophotometric properties of gilvin and that of humic acid. Gilvin (g₄₄₀, m^-1) was leached from litter primarily during the first few weeks after the commencement of the wet season. On average, 13% less gilvin was leached under ‘flooded’ conditions compared with ‘non-flooded’ conditions. Litter leaching in the presence of soil caused a significant time lag (about a month) for the peak load to occur, and reduced the total gilvin in the leachate through soil–litter interactions. Banksia menziesii leaves produced little gilvin, equivalent to 6.6% of that leached from the flooded gum. The Eucalyptus species, such as flooded gum and jarrah, common in wooded catchments in the region, appeared to be a major player producing yellow-coloured substances under annual rainfall conditions in south-western Australia.