Illusion is the second recording of the Eero Koivistoinen Quartet for Svart Records after the quartet's debut Hati Hati. Playing together for five years has made the ensemble tight and the interplay is really happening. The title tune is a new composition in a lyrical atmosphere. "Big 5" is named after five African game animals, the time signature is also 5/4. "Catalonia" is a melody from a suite El Viejo Almacen for a big band. First in rubato and then modal vamping in 6/4. "Straight Up" is one of the most popular tunes that Eero ever wrote. Their take on Miles Davis and Bill Evans' "Blue In Green" is a soprano sax and piano duet from an earlier Hati Hati session. "Hallanvaara's" melodic motives can be found in the song "Harmaaparta, Hallanvaara." It's based on a poem by Hannu Salama. The composition has a dark and free feeling starting with the famous big gong of the studio. This recording session was the last one made in the famous M1 Studio of YLE (Finnish Broadcasting Company). The closer "Goodbye M1" is dedicated to that studio.