Jamieson, Fausset and Brown on Exodus 9:27-35

27-35. Pharaoh sent, and called for Moses and Aaron, and said unto them, I have sinned--This awful display of divine displeasure did seriously impress the mind of Pharaoh, and, under the weight of his convictions, he humbles himself to confess he has done wrong in opposing the divine will. At the same time he calls for Moses to intercede for cessation of the calamity. Moses accedes to his earnest wishes, and this most awful visitation ended. But his repentance proved a transient feeling, and his obduracy soon became as great as before.