objective To evaluate the effects of payment for performance (P4P) on the availability and stockout rate of reproductive, maternal, newborn and child health (RMNCH) medical commodities in Tanzania and assess the distributional effects. methods The availability of RMNCH commodities (medicines, supplies and equipment) on the day of the survey, and stock-outs for at least one day in the 90 days prior to the survey, was measured in 75 intervention and 75 comparison facilities in January 2012 and 13 months later. Composite scores for each subgroup of commodities were generated. A difference-in-differences linear regression was used to estimate the effect of P4P on outcomes and differential effects by facility location, level of care, ownership and socio-economic status of the catchment population. results We estimated a significant increase in the availability of medicines by 8.4 percentage points (P = 0.002) and an 8.3 percentage point increase (P = 0.050) in the availability of medical supplies. P4P had no effect on the availability of functioning equipment. Most items with a significant increase in availability also showed a significant reduction in stock-outs. Effects were generally equally distributed across facilities, with effects on stock-outs of many medicines being pro-poor, and greater effects in facilities in rural compared to urban districts. conclusion P4P can improve the availability of medicines and medical supplies, especially in poor, rural areas, when these commodities are incentivised at both facility and district levels, making services more acceptable, effective and affordable, enhancing progress towards universal health coverage.