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[en] Highlights: • The role of 210Bi in the apparent secular disequilibrium between 210Pb and its granddaughter 210Po was assessed using mass balance approach. • The contribution of 210Bi to the 210Pb-210Po deficiency appears to vary with the fractionation factor and distribution coefficient. • The contribution of 210Bi to 210Pb-210Po deficiency increases with the increase in the activity ratio of particulate 210Bi to 210Pb. • The particulate 210Po sinking flux estimated from the 210Pb-210Po pair may be overestimated by as much as five times due to 210Bi sinking flux. • The 210Po derived from 210Bi may contribute more than half of the reported deficiency of 210Po with respect to 210Pb. - Abstract: The disequilibrium of the grandparent-daughter pair 210Pb (t1/2=22.3 years)-210Po (t1/2=138 days) has been used to estimate the export fluxes of particulate organic carbon in the ocean using particulate-matter-associated 210Po. 210Po is produced from 210Bi, not from 210Pb. The half-life of 210Bi (t1/2=5.01 days) is sufficiently long compared to the rates of biological particle formation and decomposition or dissolution occurring at sea. The role of 210Bi has not yet been assessed quantitatively in the apparent disequilibrium between 210Pb and 210Po, partly due to the non-existence of 210Bi depth profile measurements at sea up to now. However, greater affinity of 210Bi over 210Po and 210Pb was found recently in coastal waters and phytoplankton 207Bi uptake experiments. Build upon these findings, we developed a primitive and simple analytical approach to elucidate the role of 210Bi in the 210Po-210Pb pair in the ocean using a simplified two-box irreversible steady-state ocean model. We assumed that the activity concentrations in the dissolved and particulate phases of 210Pb, 210Bi, and 210Po in a given water column are solely determined by the concentration of the particles, their input and output, the distribution coefficients between dissolved and particulate phases, and decay constants of these radionuclides in the steady-state ocean. The 210Bi contribution to the 210Pb-210Po activity difference in seawater is found to be significant, therefore, it needs to be considered in estimating particle fluxes using 210Pb-210Po secular equilibrium at sea.