Improved methods for assessing compost maturity are needed to balance soil improvement and nutrient availability. We collected eleven separate compost samples from a single windrow over a 100 d period, beginning when the compost met minimal National Organic Standards (NOS). Our initial assessment of compost maturity used standard analyses, including total C and N, C:N ratio, and inorganic N concentration. Compost C and N lability were assessed by measuring CO₂ evolution during a 24 hr period. A commercially available compost evaluation kit (Solvita, Woods End Lab), which includes both CO₂ and NH₃ release to establish a Maturity Index, was also used. We estimated slowly degradable C fractions in the composts using neutral and acid detergent fiber (NDF and ADF, respectively) and lignin methods that were originally developed for ruminant feed analysis. The relationship between maturity (days after meeting NOS) and each compost maturity index was assessed using simple linear regression. Some widely-used parameters, like compost C:N ratio, changed very little during the sampling period, and exhibited less robust relationships with other maturity indices. Both compost CO₂ evolution and the Solvita Maturity Index showed strong linear relationships with maturity, with coefficients of determination (r²) of 0.75 to 0.85. Likewise, recalcitrant fiber and lignin concentration increased with maturity (r² = 0.75 to 0.80) during compost maturation as more easily available C was used by microbes during the composting process. The correlations between ADF, NDF, and compost CO₂ evolution were all negative (r = -0.80 to -0.86), and were slightly stronger than correlations between ADF, NDF and Maturity Index (r = 0.70 to 0.75). These results indicate that both commercial maturity indices and alternative (but commonly available) fiber analysis can be used to establish relative differences in compost maturity.