ABSTRACT: Wheat and rye bread artificially inoculated with molds were packed in modified atmospheres of 0%, 50%, 75%, or 100% CO2 balanced with N2, and 3 levels of residual O2, 1%, 0.03%, or <0.01%/O2-absorber, and stored for 30 to 35 d. Modified atmosphere packaging (MAP) was quantitatively more effective for rye bread because fewer mold species grew at elevated CO2. However, the major rye bread contaminant, Penicillium roqueforti, was the overall most CO2-resistant mold and only the use of O2-absorber could prevent growth of this species. On wheat bread, the most CO2-tolerant mold was Penicillium commune, growing in 99% CO2 (with high residual O2), and Aspergillus flavus was the mold species that grew at lowest O2 in 75% CO2 treatment. The spoilage yeast/“chalk mold”Endomyces fibuliger was less affected by the different O2 levels than the true filamentous molds, and none of the tested MAP treatments could prevent growth, but lag-phase was increased with O2-absorber on wheat bread and decreased with 1% residual O2 on rye bread. Experiments with volatile mustard oil showed that A. flavus and Eurotium repens were the most mustard oil-resistant species on wheat and rye bread, respectively. A combination strategy with MAP and mustard oil proved most optimal, and total inhibition was achieved with 2 μL mustard oil/rye bread slice and between 2 and 3 μL/wheat bread. Results indicated that the nature and surface area of the product influences effectiveness of active packaging with mustard oil.