We studied the persistence of capsicum oleoresin in a sandy loam soil. Five capsicum–soil concentrations (6.00%, 3.00%, 1.50%, 0.75%, and 0.00% wt : vol) were prepared in separate plastic boxes without lids; soils were elevated above wire-screen sub-floors to allow percolation and were exposed outdoors to ambient conditions. Plexiglass shields were placed above the boxes to allow ventilation but to deflect precipitation. During an initial 14-day period, soils were sprinkled once daily with 0.64 ml/ cm 2 of water to simulate rainfall, then were maintained dry for a final 14 days . Spectrophotometric measurements (282 nm capsaicin wavelength) of independent, dual, oven-dried, 10-g samples from each soil mixture were made for Days 0 (pre-watering), 2, 4, 6, 8, 10, 12, 14, and 28 post mixing. Results showed that: (1) simulated rainfall was an important linear regressor of capsicum oleoresin in a sandy loam soil accounting for 43–75% of spectrophotometric variation, (2) mean λ max absorbance (282 nm) of soil extracts decreased ⩾45% after cumulative sprinkling with 8.96 cm of water (14 days of 0.64 cm/ day) , then remained essentially unchanged during a subsequent 14 days without simulated rainfall, and (3) regression slopes for the initial 14-day mean λ max absorbance functions for the 4 oleoresin–soil mixtures were heterogeneous (i.e., differed in slope), with greater concentrations showing relatively faster depletion. Thus, capsicum oleoresin persisted in soil for ⩾ 28 days , with persistence enhanced under arid conditions. Although costs of capsicum oleoresin are prohibitive, these persistence data support the feasibility of developing “in-soil irritant” technology to repel fossorial rodents from prescribed areas—a less expensive, persistent irritant is sought.