Selenium adsorbed in the oxyanionic form by Fe-oxides like goethite is considered of benefit for long-term stabilization of (79)Se under near field conditions of radionuclide waste disposal sites. However, microbe-mediated volatilization of the uranium fission product (79)Se has not yet been considered for risk assessment based on the use of the water-solid distribution coefficient K(D). We have performed incubation experiments in a ternary system selenium-microbe-goethite and show that mycobiota including the common black microfungi genera Alternaria alternata are capable of volatilizing the Se even if immobilized by goethite. The microfungi were incubated in a standardized nutrient broth suspension with 10 g L(-1) of the oxide target under defined conditions. Volatile organic selenium (VOSe) species formed in the head space of the culture flasks were sampled and measured directly by a cryotrapping cryofocusing gas chromatographic system coupled with ICP-MS detection (CT-CF-GC-ICP-MS). Alkylated VOSe species were found at the tens to hundreds ng m(-3) levels dominated by dimethyl selenide (DMSe) and dimethyl diselenide (DMDSe). The total amount of DMSe released into the 80-mL headspace volume within the 21 days of incubation was up to 1.12 +/- 0.17 nmol and 0.48 +/- 0.12 nmol for systems without and with goethite amendment, respectively. Alkylation rates of up to 0.1 mumol Se per day and g biomass cannot be neglected as a potential fission product mobilization pathway, unless the inherent radioactivity is proven to prevent any such microbial activity on the long-term. Otherwise it may lead to an onsite accumulation of (79)Se through evapoconcentration in the enclosed underground caverns.