The recently introduced set of the quadratic, two-electron covalent and ionic valence indices is used to investigate the bond-breaking-bond-forming (BB-BF) process in an atom exchange reaction between H2 and X (X = H, F-I) as well as in the 0 2 -H system. Valence changes accompanying selected charge reorganizations are examined within the three-orbital model and valence diagrams for symmetric transition states (TSS) are given. The UHF valence data for Liz0 and C02 and the H-H-X, 0-0-H, and 0-H-0 (ABC) TSS (collinear and angular) are reported and compared to valence data in the separated fragments limits (SFL), AB and BC. The overall valence, v ( A B C ) , and the total (ionic plus covalent) diatomic valences, and v~c, are used as measures of the overall bond-order in a concerted BE-BF reaction, to test the postulate of the bond-energy-bond-order (BEBO) model. In collinear TSS of H2X, 7 / = -I , i.e., one bonding electron pair, is found to be roughly preserved, whereas in the angular H2X and in collinear 0-H-0 TSS, the effect of increased valence at the saddle-point is observed, relative to that of diatomic fragments (reactants or products). For the angular 0-0-H TS, a similar increase in I v ( A B C ) J relative to both 0 2 and OH s n s is detected; smaller changes relative to the 0 2 data are found in the collinear TS. This observation is in agreement with earlier predictions from the intersecting-state model. The relative diatomic valences, v: ; / v: F and %'$/-viF, are shown to conserve the overall relative bond multiplicity around 1 in both collinear and angular TSS of the H2-X systems. 0 1994 John Wiley & Sons, Inc. qo = {PEP (p} = qo.ff + qO'P. These reference occupations and the associated atodion electron populations, no = {I: 4:) (the summation is over all OAOS of atom A), define the respective occupatiodpopulation displacements: AqP = P -0 , P 9 q . ( 3 ) 0 An = nno, The explicit expressions for the QVIS are defined as follows [l]:Aq = qq , Aq" = qff -qo3",