Summary Theory and calculations are presented with respect to methane-air flamelets i viviated atmospheres. The chemical model contains 13 elementary reaction steps involving 11 species. A detailed mathematical background for the most relevant governing equations is given. The computer code READCHKIN is described in detail. The flamelet callculations were carried out for pure air and vitiated atmospheres. Three different degrees of vitiation wer treated to cover most configurations in turbulent flames in oxygen. The species concentrations, temperatures, viscocities, densities and enthalpies are given as a function of mixture fraction, ( shown in appendix E). The results for non-vitiated flamelets have been compared with the latest results from other investigators. ------- Sammanfattning Den kemiska modellen innehåller 13 elementära reaktionsteg hos 11 arter. En detaljerad matematisk bakgrund ges för de mest relevanta ekvationerna. Beräkningar gjordes för ren luft och nedsmutsad atmosfär.