Of course, it wasn't entirely the architecture of a musical statement that Mengelberg found valuable, for the personality of an artist factors in just as much, if not more. "I made a difference between the friendly and the unfriendly piano players. Monk was a very unfriendly pianist; he made fun of people and was not interested in making a nice appearance anywhere. The same went for Nichols; he was interested in form and how to get things going, but he was [also] difficult to get along with. Bebop saxophonists like Charlie Parker, whose music was similarly kinetic to that of Nichols, were nevertheless (as with Bud Powell) a 'sideline' for Mengelberg, because harmonically it did not operate the same way as Monk or Nichols. In Mengelberg's view, in fact, most saxophonists other than Lucky Thompson (and later, Dolphy and Lacy) did not understand MonkColtrane, Rollins, Rouse and Griffin all seemed an insufficient match for Monk's harmonies and rhythms, as well as his personality.
Mengelberg and Bennink were, in 1964, part of the backing group for Eric Dolphy's final studio recording, issued as Last Date (Fontana). In addition to music by Dolphy and Monk, Mengelberg contributed the composition "Hypochristmutreefuzz to the proceedings, a notoriously difficult piece with extraordinarily long intervals. "I knew he would not have enough breath to play the theme through [without circular breathing] and I thought it was funny, but for somebody who was going to die the next week maybe it wasn't right to make fun like that.
Mengelberg and Bennink were the instigators of a relatively subversive but rarely discussed small group active in Holland from 1964 to 1967. With altoist Piet Noordijk and bassist Rob Langreis, the Misha Mengelberg Kwartet toured Europe and, interestingly, played the Newport Jazz Festival in 1966 (the concert was later released by the Dutch label Artone). In 1967, Mengelberg, Bennink and reedman Willem Breuker formed the Instant Composers Pool, a loose collective of free improvisers rooted in the idea that improvisation is "instant composition , utilizing one's existing compositional knowledge to inform improvisation as well as improvisation directing the formation of composition, all in direct physical response to internal and external stimuli. Trombonist Willem van Manen, bassist Maarten Altena and altoist Peter Bennink were early collaborators, though the loose organization began to include a significant slice of the prevailing European avant-garde (including at various points Peter Brötzmann, John Tchicai and Derek Bailey). Though Breuker and his camp officially split from the ICP in the early '70s, forming the Willem Breuker Kollektief and BVHaast Records, the ICP continued as an orchestra and various aggregations, as well as the record label that in 1967 heralded the organization. The orchestra, though touring only occasionally, remains one of the most active big bands of the last 30 years, performing Mengelberg's often highly theatrical situations as well as pieces by Herbie Nichols and members of the group, which today includes reedmen Ab Baars and Tobias Delius, trombonist Wolter Wierbos, trumpeter Thomas Heberer, cellist Tristan Honsinger, bassist Ernest Glerum and violinist Mary Oliver in addition to Mengelberg and Bennink.