In the tropics the major climate determinant is rainfall. There are several potential indirect proxy measures of past relative changes in rainfall amount, the most commonly applied being changes in pollen spectra in sediment cores and inferences drawn from changing lake levels in the past. Recent work examining pollen spectra in modern sediments in lakes in the ‘Top End’ on a rainfall gradient from 800 to 1800mm indicates no relationship to rainfall due to widely varying local edaphic conditions, as well as an overprint by both fire regime and land-use. An alternative is provided by proxies in a sediment archive that record the stable isotope composition of rainfall, as water isotope composition can be related to rainfall type and the balance between rainfall and evaporation. This presentation will first examine the relationship between rainfall amount and isotope composition using a five-year daily time series of observations from Darwin. It will then examine the hydrology of Girraween Lagoon, a permanent water body near Darwin, and the impact of evaporation on the stable isotope composition of lake waters over a full seasonal cycle. Leaf waxes (n-alkanes) preserve the hydrogen-isotope composition of the waters from which they assimilated the hydrogen and are often preserved in the sedimentary record. We will present preliminary n-alkane hydrogen isotope data from the Holocene section of an 18m sediment core from Girraween Lagoon that spans the last 200,000 years. This record shows systematic variations that can be related to changes in rainfall in the Holocene. These changes can also be corroborated from the balance between dryland and aquatic pollen in the core and changes in the dryland pollen spectra. The data in combination suggest monsoon onset around ~12,700 cal BP and a mid-Holocene (~6,100 cal BP) characterized by high lake levels and low n-alkane hydrogen isotope compositions.