Heat oven to 200 degrees. Put popcorn in 2 ungreased 9 x 13" pans with enough room to stir.

Heat sugar, butter, and corn syrup, stirring occasionally until bubbly around edges. Continue cooking over medium heat for 5 minutes. Remove from heat, stir in baking soda until foamy.

Pour over popcorn; stir until well coated.

Bake 1 hour, stirring every 15 minutes.