Volatile fatty acids (VFA) C2:0 to C6:0 were used as the sole carbon source for poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHBV) production with controllable composition and microstructure in Haloferax mediterranei. Feeding carbon-even VFA gave >90 mol % poly(3-hydroxybutyrate) (3HB) PHBV, while carbon-odd VFA generated >87 mol % poly(3-hydroxyvalerate) (3HV) PHBV. Bespoke random, block, and blend copolymers with 0-100 mol % 3HV were synthesized using C4:0/C5:0 mixtures. The copolymer 3HV fraction is proportional to the %C5:0 in the feed mixture, allowing control over copolymer composition. Microstructure depends on the substrate addition order: cofeeding generated random copolymers, while sequential feeding created block and blend copolymers. On average, the PHBV had an ultrahigh molecular weight of 3 × 106 g/mol. 3HV rich copolymers showed lower melting temperatures, enhanced elasticity, and ductility. H. mediterranei is ideal for large-scale production of PHBV due to its inherent bioprocessing advantages, while control over the composition and microstructure of PHBV will facilitate the production of biopolymers capable of meeting industrial criteria for specific applications.