Earlier I indicated that I kept a container of the dpd powder for over one year before degradation. The stuff I have now I bought two years ago and lasted two full seasons, with what appear to be accurate results (add 5 ppm of bleach, get a 5 ppm rise in measurement). I keep it in a dry location on the bottom floor of the house. I do not put my test vials in the kit container so there is no moisture locked in. I am looking forward to Al's report on the stuff he keeps in the fridge.