Why does cold coffee taste sweeter with the same amount of sugar?

This research paper suggests that, in general, pH, temperature, and ions don't generally significantly affect our perception of sweetness intensity. Since temperature and acidity are the two major differences between iced and hot coffee (depending on preparation), this more or less rules out the possibility of a chemical difference.

"The main finding from this three-part study was that temperature, pH, and ions had little effect on the perceived sweetness intensity of the sweeteners studied. Even when significant differences were found in the temperature study, the effects were very small."

This, to me, suggests it's either due to larger "sips" of cold coffee and more of an opportunity to taste the sweetness or because sugars dissolve less readily in the cold coffee and it's easier to perceive the sweetness when it's not in a solution.

Its possible that when you drink a hot liquid, you swish it around your mouth a lot less than when you drink a cold liquid, and thus fewer of the taste buds on your tongue are exposed to it. This wouldn't be surprising: it's hot after all.

Another possibility is that if you're starting with hot coffee, and adding sugar to it, that the sugar isn't fully dissolved when you drink it hot (it takes time, especially if you don't stir it much), but has dissolved by the time the coffee cools. With more dissolved sugar, it is sweeter.

I suggest the change is in the coffee rather than the sugar. Coffee is more aromatic when hot. When you put the same amount of sugar in a hot and cold cup of coffee, the sweetness will dominate more in the cold coffee where there the coffee taste/aroma is less pronounced.

Actually, it has nothing to do with the solubility of the sugar. The reason is that fructose (in an aqueous solution) has two different cyclic forms. The Beta-fructopyronose form (6 membered ring form) is much sweeter than the furanose (5 membered ring) analog. At lower r temperatures the equilibrium shifts and more of the pyranose-ring fructose is present (converted from alpha and beta fructofuranose) thus increasing the perception of sweetness.

Cold coffee tastes less bitter than hot coffee. When I was a kid I worked in a bakery. The last pot of coffee was always left for the first bakers, who came in at 3 AM. By the time I got there at 5 AMthe coffee was at least 12 hours old and on a burner for at least a couple of hours.

I found that if you put some of the coffee over ice it tasted OK. My favorite coffee drink today is ice with a couple of shots of espresso. No sugar needed.

Sweetness perception of coffee-based foods is different from most other foods, since coffee naturally contains the compound lactisole which has a more or less negative sweetness - temperature dependent effects can be from differences in solubility, viscosity etc...