* By statistical weight we mean the factor that a segment contributes to the partition function, including, if appropriate, a Boltzmann factor; it is not just the number of quantum states, in contrast to one popular usage.

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The theory of the interaction of nuclear electric quadrupole moments with molecular rotation is extended to asymmetric-top molecules containing one or two quadrupolar. nuclei. The firstorder theory is developed in detail, and in a form which makes use of published numerical tables of line strengths of asymmetric-top pure rotation transitions, and involves the second derivatives of the electrostatic potential V taken along the three principal axes of inertia. Only two of these are independent, and (d 2 V/dz' 2 ) and (d 2 V/dx' 2 )-(d 2 V/dy' 2 ) are a convenient choice of the parameters of the problem. A general formulation of the theory, which includes higher order effects, is given. In case a molecule contains only one quadrupolar nucleus, the structure of the quadrupole multiplets should be sufficient to identify pure rotation transitions.