Pop the popcorn and set aside.

In a large saucepan, combine margarine, brown sugar, and corn syrup.

Bring the mixture to a boil, stirring occasionally to prevent scorching.

Boil for 5 minutes, continuing to stir.

Remove the saucepan from the heat.

Add baking soda; the mixture will foam up.

Pour the caramel over the popped popcorn and mix well to coat evenly.

Pour the coated popcorn into an unbuttered large pan (a roaster works well).

Place the pan in a preheated 200°F (93°C) oven for 1 hour, stirring approximately every 15 minutes to prevent sticking.

Remove from the oven and allow to cool slightly.

Store the caramel popcorn in a covered container to maintain freshness.