A substantial reward will be offered in return for information leading to the recovery of the three Jackson Pollock paintings stolen last Thursday from the Cambridge apartment of Reginald R. Isaacs, Norton Professor of Regional Planning.

The company which insures the three paintings will decide today or tomorrow on the amount of the reward. The paintings are valued at an estimated $500,000.

The three stolen paintings were a Pollock black and white called "No. 7, 1951"; an unnamed 1948 work from his drip period; and an untitled 1949 work, also from the drip period.

Cambridge police and federal investigators continued their search yesterday for two men who were seen placing the stolen canvases in a dark-colored car about 4:40 p.m. Thursday afternoon.

A woman apparently saw the two suspects carrying the paintings down the fire escape at the east end of the condominium complex where the Isaacs live.

Isaacs was rowing on the Charles River when the theft occurred. He left his apartment for the Cambridge Boat Club at 3:15 p.m. and returned at 4:45 p.m.

When he returned to the apartment, he discovered the paintings gone and an empty briefcase in the living room.

The value of Pollock's works skyrocketed recently after the $2 million sale of Pollock's "Blue Poles" to the Australian National Museum.