A series of subcritical experiments in unreflected annular geometry was performed with an aqueous Pu-U nitrate containing 173 and 262 g/ℓ of plutonium and uranium, respectively. The plutonium contained 91.1 wt% 239Pu, while the depleted uranium contained 0.57 wt% 235U. In these experiments, the height of the solution in the annulus was varied from 17.8 to 84.2 cm. The annulus had an inner diameter of 25.4 cm, an outer diameter of 53.34 cm, and a 0.08-cm-thick wall of Type 304L stainless steel. Measurements using the 252Cf-source-driven neutron noise analysis method were interpreted to obtain the subcritical neutron multiplication factors. The data accumulated in the experiment, which is the first test of this method in annular geometry, are summarized, and the analysis of these data is presented. The results and conclusions of these experiments are as follows: (a) the capability to measure the sub-criticality for a multiplying system of annular geometry to a k as low as 0.70 was demonstrated; (b) the criteria developed in previous experiments for choosing source-detector system configurations for which the data can be interpreted using a modified point kinetics were also satisfactory for this experiment; (c) the measurement times for this geometry were not significantly different from those used for cylindrical geometry and were sufficiently short to allow practical measurements; (d) the reactivities obtained from break frequency noise analysis measurements agreed with those obtained from the ratios of spectral densities within the experimental uncertainties; (e) the applicability of the method and an understanding of the theory of the measurement method for plutonium solution systems were demonstrated; and (f) the calculated neutron multiplication factors agreed with the experimental values of k to within ∼0.03.