Potato starch can be substituted for flour when making roux:

1 tablespoon of potato starch + 1 tablespoon of butter/fat will thicken 1 cup of liquid.

Potato starch can lose its thickening power if brought to a boil for too long. Make sure to monitor the heat and simmer the soup or sauce. Potato starch is an ideal substitution for people who are gluten intolerant.