NOVELTY - Process (P1) for forming low-acid biomass-derived pyrolysis oil involves: pre-treating biomass-derived pyrolysis oil; and esterifying oil in presence of supercritical alcohol and catalyst composition of material selected from unsupported solid acid/base catalyst and catalytic metal dispersed on metal oxide support. The solid acid catalyst comprises molecular sieve and Group V metal oxide. The molecular sieve comprises zeolite and MCM 41. The base catalyst comprises e.g. calcium oxide; catalytic metal comprises e.g. noble metals and metal oxide support comprises e.g. Group IV metal oxide.    USE - For producing low-acid biomass-derived pyrolysis oil (claimed) used as fuel for certain boiler and furnace applications, and as a potential feedstock in the production of biofuels in petroleum refineries or in stand-alone process units.    ADVANTAGE - The method reduces the total acid number (TAN) of the biomass-derived pyrolysis oil to form low acid biomass-derived pyrolysis oils. The method produces low acid biomass-derived pyrolysis oils having increased energy density, thermal stability and lower acidity as compared to conventionally-produced biomass-derived pyrolysis oil. The processes are relatively inexpensive and simple to implement.    DETAILED DESCRIPTION - A process (P1) for producing a low-acid biomass-derived pyrolysis oil involves: pre-treating a biomass-derived pyrolysis oil to form a treated acid-containing biomass-derived pyrolysis oil; and esterifying the treated acid-containing biomass-derived pyrolysis oil in the presence of supercritical alcohol and a catalyst composition to form the low-acid biomass-derived pyrolysis oil, where the catalyst composition comprises a material selected from an unsupported solid acid catalyst, an unsupported solid base catalyst, and a catalytic metal dispersed on a metal oxide support, where: the unsupported solid acid catalyst comprises a material selected from a molecular sieve and a Group V metal oxide. The molecular sieve comprises a material selected from zeolite and MCM 41. The unsupported solid base catalyst comprises a material selected from an alkaline earth metal exchanged molecular sieve, calcium oxide (CaO), magnesium oxide (MgO), and silicon oxide (SiO2). The catalytic metal comprises a metal selected from noble metals, and/or non-noble metals, and the metal oxide support comprises a metal oxide selected from a Group IV metal oxide, a Group V metal oxide, and/or Group IIIA metal oxide. An INDEPENDENT CLAIM is included for a process (P2) for producing a low-acid biomass-derived pyrolysis oil involving: pre-treating a biomass-derived pyrolysis oil to form a treated acid-containing biomass-derived pyrolysis oil; diluting the treated acid-containing biomass-derived pyrolysis oil with supercritical ethanol to form a solution including at least about 30 wt.% ethanol; and esterifying the treated acid-containing biomass-derived pyrolysis oil included in the solution in the presence of a catalyst composition and in the absence of gas in an up-flow reactor to form the low-acid biomass-derived pyrolysis oil, where the catalyst composition comprises a noble metal support on a sulfated metal oxide.