The demand for renewable energy and chemicals from biomass resources has led to the development of methods for biorefining lignocellulosic materials into pellets and value-added chemicals. This has, in turn, driven a search for alternative raw materials to use as feedstock in such biorefineries. This study proposes the inclusion of low-grade lignocellulosic residues from agriculture and forestry, such as straw and woody biomass with a high bark content, in the feedstock mix for steam explosion (STEX) based biorefineries. It focuses on exploring how the use of low-grade residues affects the yield of value-added chemicals that arise as side streams during the steam explosion of lignocellulosic biomass and if acetic acid impregnation of the biomass prior to STEX pretreatment will further improve yields of valuable side stream chemicals. The raw materials were impregnated with water/acetic acid before being subjected to STEX pretreatment, using high temperatures (190 °C–223 °C), and a reaction time of 8 min before pressure release. Value-added chemicals were identified in two separate aqueous side streams. In the condensed process effluents, the most abundant compounds were acetic acid (1.2 g*kg−1 dry input biomass) and furfural (1.4 g*kg−1 dry input biomass), and the highest yields were found in the samples from mixed spruce and branches and tops (BRAT) at high temperatures. Filtrate samples produced from washing the steam-exploded biomass contained acetic acid (53.6 g*kg−1 dry input biomass), furfural (4.1–7.9 g*kg−1 dry input biomass) and 5-hydroxymethylfurfural (5-HMF) (9.0 g*kg−1 dry input biomass) as the most abundant compounds and highest yields of target compounds were found in samples of spruce alone and of spruce mixed with BRAT. All compounds in the side streams were identified and quantified using quantitative 1H nuclear magnetic resonance spectroscopy (qNMR). This study demonstrates that high yields of valuable chemicals are present in the aqueous side streams of pellet production using alternative feedstocks such as forestry and agricultural residues.