We identified an apparent seasonal pattern in lead release to orthophosphate-treated drinking water via point of use sampling. And while variation in orthophosphate, pH, and alkalinity predicted a similar pattern in equilibrium lead solubility, seasonal variation in aluminum may have also been a factor, given its correspondence with observed lead levels. In a follow-up coupon corrosion study, aluminum increased total lead release significantly. As expected, orthophosphate decreased lead release, but high levels of aluminum and orthophosphate together resulted in greater lead release than would be predicted based on the main effects of these two factors. We suggest that the interference of orthophosphate corrosion control by aluminum is due largely to precipitation of aluminum phosphate. This reaction limits the activity of orthophosphate and may provide a surface to which soluble lead can partition-increasing the total lead content of drinking water.