The rate constants for the reaction of hydroxide ion with syn- and anti-9,10-dioxabimanes (1,5-diazabicyclo[3.3.0]octadienediones) correlate with the function [σ (R1) + 0.5σ (R1′)], ρ value of 3.0 (R2 = CH3 or H), and with [σ (R2) + 0.5σ (R2′)], ρ value of ca. 4 = Cl or H). The ρ value for hydrolysis of lactones produced by photoisomerization of anti-bimanes is 3.7. Structures of the ring-opened hydrolysis products as 1(2)-pyrazolinonylacrylic acids have been confirmed for both syn- (2-acid) and anti-bimanes (1-acid). The E configuration is established by 1H NMR for the side chain of the hydrolysis product of syn-(H,H)B. Identical ring-opened acids are obtained from anti-bimanes and the lactones. The ring-opened acids are cyclized readily to the anti-bimanes and/or lactones; some data on the facility of the conversion are presented. The present study provides information valuable for improving the yields of bimanes and the design of biological labeling experiments.