Three reactions have been studied by the infrared chemiluminescence method: (1a) H + ICl → HCl( v′ ⩽ 7) + I, (2a) H + BrCl → HCl( v′ ⩽ 7) + Br and (2b) H + BrCl → HBr( v′ ⩽ 4) + Cl. The detailed rate constants k( V′, R′, T′)( V′, R′, T′are product vibrational, rotational and translational energies) have been obtained for all three reactions. For (1a) the results confirm and amplify earlier results obtained by the same method. The overall average fractions of the total available energy from (1a) entering vibration, rotation and translation in the HCl are f′ V = 0.59, f R = 0.21. f′ T) = 0.20. The reaction exhibits “ microscopic branching” into two separate HCl product-energy-distributions, the so-called “low- J” and ”high- J“ components, for which f V′ Q 0.36, f R′ Q = 0.04, f R′ Q = 0.60, and f V′ h)= 0.65, f R′ h = 0.24 and f T′ h = 0.11. The microscopic branching ratio k h/ k Q = 4.56. The HCl product of reaction (2a) was formed with an overall average of f′ V = 0.55, f′ R = 0.09 and f′ T = 0.36. There is clear evidence of microscopic branching in the product energy distribution from reaction (2a), but the branches lead to overlapping energy-distributions. The microscopic branching ratio k h k Q ≈ 1.6. For reaction (2) both of the macroscopic branches could be studied by infrared chemiluminescence. The HBr product from reaction (2b) had f′ V = 0.58, f R) = 0.12 and f′T) = 0.30. There was no evidence of microscopic branching in (2b). The same considerations applied to reactions (1a), (2a) and (2b) provide a framework for understanding the present findings. If approach from the Cl end of ICl or BrCl has a higher energy barrier, then migratory reaction from the 1 or Br end provides an energetically favoured indirect route for formation of HCl product [the k h microscopic branch in (1a) and (2a)]. Direct reaction at the Cl end is, on this view, responsible for the second microscopic branch ( kℓ). There is no energetic advantage to forming HBr in (2b) by migration from the Cl, consequently a single reaction mode is observed. The same non-migratory dynamics is predicted for (1b) (→ Hl). More generally the prediction is that for a reaction H + XY two competing microscopic reaction paths (direct and migratory) will exist (only) for the halide product, HX or HY, that contains the more electronegative of the halogen atoms. Microscopic branching was shown to stem from causes other than reaction onto alternative potential-energy surfaces, leading to X( 2P in3 2 ) and X*( 2P 1 2 ), since the fractional yield of I* and Br* was found to be orders-of-magnitude smaller than k Q/ k h. Summation of detailed rate constants for reaction (2a) and for (2b) gave a value for the macroscopic branching ratio ▪ = 0.40. This was lower than the ratio obtained by the application of simple information theory to the product energy distributions from (2a) and (2b); ▪ 3.2. It appears that the difference in activation barrier for approach from the two ends of BrCl, postulated in connection with the microscopic branching, may also be the dominant factor in determining the macroscopic branching ratio.