The concept of kerma (K) is useful in understanding the two-step process by which indirectly ionizing radiation deposits energy in matter. This quantity becomes even more important in fulfilling this role, and in relating the quantities of radiological physics to each other, if one subdivides K into its collisional part (Kc) and its radiative part (K,), according to the fate of the kinetic energy transferred to the secondary electrons. Such a partitioning is only important for × and y rays, since K, = 0 in all practical cases for neutrons. The exposure can then be precisely defined as (Kc)air (e/W)air The relationship of K, Kc and D is particularly interesting for high-energy y rays where only transient CPE can be attained: Then K/Kc = microtr/microen and D = Kc(1 + microx), where micro is the effective attenuation coefficient of the y rays and × is the mean distance the electrons carry their kinetic energy “down-stream” before depositing it as dose, as defined by Roesch. As a result D usually remains less than K in the transient CPE region, contrary to what is commonly supposed, since microtr,/microen exceeds (1 +microx) in most cases. It is also shown that Roesch's relation of D to Kc, and thence to X, provides a theoretical basis for the determination of exposure for higher gamma-ray energies than the accepted 3-MeV limitation.