Although Ukraine was one of the three founding countries and ratified the Creation Agreement in December 1991, Ukraine did "not" choose to ratify the CIS Charter and thus does not regard itself as a member of the CIS.
Although Ukraine was one of the three founding countries and ratified the Creation Agreement in December 1991, Ukraine did "not" choose to ratify the CIS Charter as it disagrees with Russia being the only legal successor of the Soviet Union. Thus it does not regard itself as a member of the CIS.