Studies have shown that debromination of the major component in the deca-brominated diphenyl ether mixture (deca-BDE), BDE-209, occurs in vivo in birds. Recent work from our laboratory on breeding ring-billed gulls (Larus delawarensis) exposed to elevated PBDE concentrations in the densely-populated metropolis of Montreal (Canada) further suggests that BDE-209 debromination is potentially catalyzed by deiodinases in liver microsomes. The first objective of this study was to determine if type 1 deiodinase (D1) was involved in the in vitro debromination of BDE-209 in liver microsomes of ring-billed gulls. The second objective was to determine if there was an interaction between D1 and BDE-209 using an in vitro D1 activity assay. No depletion of BDE-209 was observed in gull liver microsomes. A significant 42% increase in total D1 activity was found in gull liver microsomes at the medium BDE-209 concentration (1.0 nM), although not at the low (0.5 nM) or high (2.5 nM) concentrations, suggesting potential non-dose related interaction with D1. Moreover, no correlation was found between total D1 activity in liver microsomes and plasma thyroid hormone levels, although there was a negative relationship between plasma BDE-209 concentrations and FT3 levels. Results from this study suggest that debromination of BDE-209 did not occur using present in vitro assay conditions, although indicated potential interaction with D1 that may have implication on circulating thyroid hormone status.