Khodorkovsky, 42, currently serving an eight-year sentence for fraud and embezzlement in Siberia, was put in solitary confinement for the second time on March 17 for drinking tea in an unauthorized place.
A local court ruled last week that a decision to place the oligarch in isolation for the first occasion on January 24 for allegedly possessing inappropriate documents had been incorrect.
"If the first placement of Mikhail Khodorkovsky in an isolation cell was illegal, then the second placement was also without foundation," said Semyon Rosenberg, a lawyer who represents Khodorkovsky's interests in court.
The former Yukos head was placed in an isolation cell for the third time on June 5 for purportedly breaching a prison regulation prohibiting inmates from "selling, buying, presenting, accepting, or seizing personal food products, objects, or substances" after a three-day meeting with his wife.
