Estimates of monsoon intensity in southern China during the Oligocene are limited. Here we report new estimates of wet season rainfall based on stable oxygen isotope measurements of cellulose extracted from living Pinus massoniana trees from Qingxiushan Hill, Nanning (22°47'23.35” N, 108°23'4.26” E, elevation = 223 m) and fossil Castanopsis wood from the upper Yongning Formation near Nanning in the Guangxi Zhuang Autonomous Region of South China (22°52'50'' N, 108°25'2'' E). The modern data span 11 years (1990-2000) from two radial tree cores (QXS21A and QXS24A) for calibration with modern climatological data; 64 mummified wood samples dated to the late Oligocene were selected for cellulose extraction, but only 43 had recoverable cellulose. Cellulose was extracted following the modified Brendel method and oxygen isotope values were determined using a High–Temperature–Conversion Elemental Analyzer coupled with a Delta–V Advantage Mass Spectrometer. Values are reported in units of per mil (‰) relative to the VSMOW standard, with an analytical uncertainty of ± 0.3‰ and ± 0.1‰ for the fossil and modern samples, respectively.