I was told several years ago that A&H soda was the ANHYDROUS form, and thats how I've been using it for at l;east 2 years! Last week I called A&H and asked them, and they told me it was monohydrate. It would not be a bad idea if someone else called them, or even several people (the number is on the box) because these days you never know how many different answers you might get.(posted 8442 days ago)I've never heard of decahydrate carbonate, but I'm also a chemistry flunker. Are you sure you arent thinking of 20 Mule Team borax, which is decahydrate?