Speleothems provide paleoclimate information on multimillennial to decadal scales in the Holocene. However, seasonal or even monthly resolved records remain scarce. Such records require fast-growing stalagmites and a good understanding of the proxy system on very short timescales. The Proserpine stalagmite from the Han-sur-Less cave (Belgium) displays well-defined/clearly visible darker and lighter seasonal layers of 0.5 to 2 mm thickness per single layer, which allows a measuring resolution at a monthly scale. Through a regular cave monitoring, we acquired a good understanding of how δ 18 O and δ 13 C signals in modern calcite reflect climate variations on the seasonal scale. From December to June, outside temperatures are cold, inducing low cave air and water temperature, and bio-productivity in the soil is limited, leading to lower p CO 2 and higher δ 13 C values of the CO 2 in the cave air. From June to December, the measured factors display an opposite behavior. The absence of epikarst water recharge between May and October increases prior calcite precipitation (PCP) in the vadose zone, causing drip water to display increasing pH and δ 13 C values over the summer months. Water recharge of the epikarst in winter diminishes the effect of PCP and as a result the pH and δ 13 C of the drip water gradually decrease. The δ 18 O and δ 13 C signals of fresh calcite precipitated on glass slabs also vary seasonally and are both reflecting equilibrium conditions. Lowest δ 18 O values occur during the summer, when the δ 13 C values are high. The δ 18 O values of the calcite display seasonal variations due to changes in the cave air and water temperature. The δ 13 C values reflect the seasonal variation of the δ 13 C DIC of the drip water, which is affected by the intensity of PCP. This same anticorrelation of the δ 18 O versus the δ 13 C signals is seen in the monthly resolved speleothem record that covers the period between 1976 and 1985 AD. Dark layers display lower δ 18 O and higher δ 13 C values. The cave system ...