People sometimes wonder what the difference is between “regular” cocoa and Dutch-processed cocoa. Well, according to Wikipedia, Dutch-processed cocoa “is less acidic, darker and more mellow in flavor than what is generally available in most of the world. Regular (non-alkalized) cocoa is acidic.” So if you substitute one for another, there might be a difference in the pH of your baked goods, resulting in a difference in rising, for example. When a recipe specifies one type or the other, this has already been taken into consideration. However, I must admit that I usually only keep one type on hand at any given moment, and I don’t bother compensating – no one complains.
For reference, in the picture above, the cocoa on the left is Fry’s cocoa, a regular-type cocoa that seems to be the most widely available in supermarkets. On the right is Van Houtten, a Dutch-processed cocoa – notice how much richer the color is!