Differential scanning calorimetry and wide-angle X-ray diffractometry first revealed the formation of hetero-stereocomplex (HTSC) between biodegradable, optically active, and isotactic poly(2-hydroxyalkanoic acid)s having different chemical structures and opposite configurations, i.e., L-configured substituted poly(lactic acid) (PLA) [poly(L-2-hydroxybutanoic acid), P(L-2HB)] with linear side chains (ethyl groups) and D-configured substituted PLA [poly(D-2-hydroxy-3-methylbutanoic acid), P(D-2H3MB)] with branched side chains (isopropyl groups) in solution and in bulk from the melt. The melting temperature of P(L-2HB)/P(D-2H3MB) HTSC crystallites was 197-204 degrees C, which is much higher those of P(L-2HB) and P(D-2H3MB) homo-crystallites (100-101 degrees C and 158-165 degrees C, respectively). The interplain distances and crystalline lattice sizes of P(L-2HB)/P(D-2H3MB) HTSC crystallites were respectively larger and smaller than those of P(L-2HB)/P(D-2HB) and P(L-2H3MB)/P(D-2H3MB) homo-stereocomplexes. The HTSC formation of substituted PLA with opposite configurations reported in the present study will provide a versatile way to prepare poly(2-hydroxyalkanoic acid)-based biodegradable materials having a wide variety of physical properties and biodegradability. (C) 2014 Elsevier Ltd. All rights reserved.