Hypothesis: TiO2-NPs-Cellulose composites functionality depends on the retention and dispersion of NPs in the composites. SAXS and SEM can be combined to reveal the effect PAE has on the NPs aggregation, retention and interaction mechanisms in the composites. Experiments: TiO2-NPs-Cellulose sheets were made by first preparing PAE-cellulose suspensions of different PAE dosages (10 and 50 mg of PAE/g fibres). The TiO2 NPs suspension (at different NPs loading) was then added to the cellulose-PAE suspension. The final suspension was used to make flexible paper-like composites sheets. SEM and SAXS quantified NPs retention and aggregation state. Findings: PAE dosage of 20 mg/g cellulose provides full surface coverage of cellulose fibres. A 10 mg of PAE/g cellulose covers half the cellulose surface area and no free PAE remains in the suspension. PAE dosage of 50 mg/g cellulose gives full cellulose surface coverage and provides a large amount of PAE (30 mg/g cellulose) free in the suspension. Surprisingly, at both PAE dosages, NP coagulates and the size of the aggregates increase with NPs loading. Aggregates of two particle sizes (10 and 35 nm) are formed and the number density of smaller particles is higher than larger particle. The NPs aggregates and their retention are similar at both PAE dosages, which is explained by different PAE-NPs bridging mechanisms. (C) 2020 Elsevier Inc. All rights reserved.