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Graphical abstract Highlights • For studying degradation kinetics of complex lignocellulosic papers, the cellulose peak (C) in multimodal the MMD was deconvoluted. • Two deconvolution methods which use SEC-MALS data are proposed to determine M r without the disturbance of HC or LCC fractions. • M r , k and E a close to cotton linter paper were obtained with the deconvolution method based on a calibration curve built with pure cellulose. Abstract Complex cellulosic samples are often difficult to analyse with size-exclusion chromatography. The strong molecular associations of hemicelluloses and lignin with cellulose produce multimodal molar mass distributions (MMD) that are difficult to interpret. More reliable ways of calculating the molar masses of cellulose are thus necessary. This is particularly relevant when studying the kinetics of paper degradation, as the number average molar mass is the most precise indicator. In this study various data handling methods based on the deconvolution of bimodal and multimodal MMDs of complex cellulosic samples after SEC-MALS-DRI analysis are examined in order to propose more accurate paper degradation rates. Two deconvolution methods, which do or do not rely on polymer calibration curves were developed and were applied to several kraft and groundwood pulp papers unaged and hygrothermally aged. The deconvolution methods are discussed and evaluated in light of calculated cellulose activation energies, degradation rates and paper usable lifetime predictions. [ABSTRACT FROM AUTHOR]