Materials Purchased

My country is going into debt because of "Materials Purchased".

The hover-help says "This is the cost of material, mainly consumption goods that were purchased by the country for use by the population. This should not be seen as government cost. It is purchased for the population and is paid for by the population"

I beg to differ about that last part - this month I just forked out $84M for Materials Purchased and it turned a $20M profit into a $65M deficit for the month.

Any idea what it is, and how to stop it?

If I'm not mistaken, that would come out of their Tax Dollars, in which you have to pay, but practically, they're paying for it. The population uses a variety of things, for example, electric power, cable TV, services, food products, etc. The only way you can help reduce it is either, by reducing your population (Not Recommended), or by making companies to provide the products you need (Which won't necessarily reduce the cost, but just keep of a steady flow of products, which will prevent riots), or by joining a Common Market (Which is similar to the previously stated topic). You really can't reduce the materials purchased, but I recommend that you, instead, increase your revenue, and decrease your spending. You can do this by deactivating certain arms, both offensive and defensive. You can also increase your revenue by creating companies that specialize in the creation of products that have a large deficit in the in

You need to give country name and world if you want someone to take a look.

Ok, first of all that tooltip isnt completly right.
The best way to decrease you material cost is to reduce the Quality ordered. Per default your Counry buys any Quality it gets but the Population only pays for Quality 120 or something about that. So any higher Quality you/your Country buys is lost Money for now.
When you get to a more stable economy you can increase the Quality again as it slightly increases your Welfare index but for now I'd recommend to decease it to 120.

you need to look at these figures long term. Your country might have bought 100M of materials at once but the population don't use and pay for it all in one go. Looking at the year to year numbers will show you that the pop do indeed pay the country back.