Here is an official resolution which explains all.

On the basis of expert analysis by the Ukrainian Language Institute under the National Academy of Sciences of Ukraine regarding the Roman-letter correspondence to the Ukrainian language geographic name of Kiev, taking into account that the spelling Kyiv is indeed in the modern practice of Ukraine’s international communication, proceeding from the urgent need to standardize the recreation of Ukrainian proper names through Roman letters tn the context of Ukraine’s integration into the world legal realm, based on point 6 part 4(b) of the Provision on the Ukrainian Commission for Legal Terminology approved by decree No 796 of the President of Ukraine on August 23, 1995 "Regarding the Provision on the Committee for Legislative Initiatives under the President of Ukraine, on the Ukrainian Codification Commission and on the Ukrainian Commission for Legal Terminology", the Commission HAS APPROVED:

1. To acknowledge that the Roman spelling of Kiev does not recreate the phonetic and scriptural features of the Ukrainian language geographical name.

2. To confirm that spelling of Kyiv as standardized Roman-letter correspondence to the Ukrainian language geographical name of Êè¿â.

3. On the basis of point 7 of the Provision on the Ukrainian Commission for Legal Terminology, determine as mandatory the standardized Roman-letter spelling of Kyiv for use in legislative and official acts.

4. The resolution to be effective from the moment of its approval.

Head of the Commission,
Minister of Justice of Ukraine -- S. Holovaty
Chief Secretary of the Commission -- Y. Zaitsev