One usually assumes that Sjöstrand’s area method for determination of reactivity by the pulsed-neutron technique is only valid in case of exponential prompt-neutron decay and no kinetic distortion. In this paper the method is shown to be valid more generally. Namely, for all systems satisfying multigroup multinode reactor equations, with only one fissioning node, the method holds true exactly when reactivity is understood to be the static reactivity, while βeff is defined as the relative difference between the static prompt and total multiplication factors, provided the sensitivity of the detector has the same energy dependence as the fission cross section of the fuel of the active zone. It follows, e.g., that Sjöstrand’s method with a suitable fission counter is very well suited for measurement of subcriticality in small reflected subcritical fast cores. Some general recommendations are given.