Combine popped popcorn and peanuts in a large roasting pan; keep warm in a 300 oven while preparing syrup.

Combine brown sugar, butter, corn syrup and salt in a heavy 2 quart saucepan. Bring to boiling over medium heat, stirring almost constantly. Boil gently over entire surface for 5 minutes, stirring occasionally. Remove from heat; stir in baking soda. Pour over popcorn and peanuts. Mix well.

Bake in a 300 oven for 20 minutes, stirring once after 15 minutes. Immediately spread out popcorn mixture on a large piece of foil; cool completely. Break into cluster. Store tightly covered. Makes about 11 cups.