Data on lacustrine sediments permitted reconstructing climate and vegetation changes in the steppe zone of southeastern Transbaikalia in the Holocene and the evolution of Lake Zun-Soktui. Also, changes in the size of the lake water plane have been reconstructed. A shallow lake formed in place of an oxbow lake in the middle Boreal phase of the Holocene. In the Atlantic phase, the lake had maximum depth and area. In the late Boreal, early Subboreal, and in the second half of the Subatlantic phase, the lake underwent significant shallowing and sometimes dried up for a short time. In the middle Boreal and Atlantic phases, the aridization decreased considerably. It has been established that pine forest belts on the Onon high plain before the Subatlantic occupied several times more space than now.