Like everything else, it is determined by the supply-demand ratio, and since the supply is fixed (well, it changes, but at a known, fixed rate), it is basically determined by demand. More specifically, it is determined by all the standing bid/ask orders at the exchanges, and since MtGox handles 80% of that trade, it is basically determined by the bid/ask orders at MtGox. Take a look at this: http://mtgoxlive.com/orders The green line is the USD exchange rate for a bitcoin, and the orange and blue lines show how much BTC would have to be converted to USD, or vice versa, in order for the exchange rate to change to that point. The value of BTC listed on SR depends on where they are scraping the info. That source could lag behind MtGox by several hours (or SR may only scrape every few hours), so it will be slightly different.