[vetinari]

The 1991 referendum was: Do you support re-establishing the Crimean Autonomous Soviet Socialist Republic as a subject of USSR and a party to the Treaty on the Creation of the USSR? That means to be on the same level as Ukraine in the USSR, not under Ukraine.

There were additional referendums, in 1992 and 1994, but they were asking questions with the reality of being part of Ukraine.

[valcker]

Crimea was an "oblast" (or region) of Ukrainian SSR and the referendum was about re-establishing the Crimean Autonomous republic which was the case and was signed by Ukrainian SSR council: http://zakon1.rada.gov.ua/laws/show/712-12

During the separation of republics from USSR, Crimea stayed as a part of Ukraine (and voted for the independence of Ukraine, btw) which was a logical move in terms of infrastructure.