They are never safe to use in the microwave, as the heat releases chemicals. They are a different type of plastic than the micro-safe ones. I personally wouldn't use them at all, especially for longer-term food storage. My mom used to when I was a kid, and they always leaked if we had liquidy foods in them. Storage contianers meant for leftovers, ect. are not that expensive-you can get a box of 20 containers or so for $10, and they'll last a lot longer. We're actually getting glass containers, but they cost more, and we're purchasing them gradually.