1. Direct measurements of water-table depth in a mire were compared with depth to discolouration of polyvinyl chloride (PVC) tape left in situ for 15 months. Discolouration depth was highly correlated with positions of highest and lowest water tables; estimates were systematically biased but generally within 5 cm of measured values.
2. The annual range in water-table position indicated by discolouration depth varied 10-fold along a 30 m transect. Discolouration of PVC tape is an indicator of reducing conditions that could be applied in extensive surveys at very low cost.