Compression in the through-thickness direction (as in fastening) resulted in reversible and irreversible changes in the microstructure of continuous carbon fiber epoxy-matrix composites, as shown by electrical resistance measurement during dynamic compression. The extent of fiber-fiber contact across the interlaminar interface was increased, with partial irreversibility even at a low stress amplitude of 1 MPa. Within a lamina, fiber squeezing in the through-thickness direction and fiber spreading in the transverse direction occurred upon fastening compression, with partial irreversibility at a stress amplitude of 100 MPa or above. For a single laminae beyond 400 MPa, the lessening of fiber squeezing in the through-thickness direction during unloading dominated over the fiber spreading in the transverse direction during loading. (c) 2006 Springer Science + Business Media, Inc.