Paraformaldehyde is the condensation product of methylene glycol, a substance not so far isolated in its pure form; it is a white solid with a formaldehyde content between 78 and 98% and is supplied either as a Prill or Granular. All forms contain more than 95.5% paraformaldehyde content. The Prill and Granular forms are small, free-flowing, dust-free beads, the most desired by EM users. A fast-penetrating EM fixative used in combination with Glutaraldehyde, Acrolein, and Osmium Tetroxide. Karnovsky, M.J., J. Cell Biol., 27, 137A (1965). Hayat, M.A. Principles & Tech., of EM (Univ. Park Press).