The regioselectivity of the [2 + 2] photocycloaddition reaction between triplet pi pi* acrolein and substituted olefins in their ground states was studied using the reaction force concept and reactivity indices from conceptual spin-polarized density functional theory. In the first part, the reaction path was determined for the attack of the acrolein alpha- and beta-carbon atoms on the alkenes, yielding biradical intermediates evolving to the head-to-tail (HT) and head-to-head (HH) regioisomers, respectively. The beta pathway was found to be the most favorable path from the thermodynamic and kinetic points of view, indicating that the formation of the HH cycloadduct should be preferred for reactions with both electron-rich and electron-poor alkenes if this first step determines the final regioselectivity. In the second part, the reactivity of the biradical intermediates was characterized through global and local spin-polarized response functions, together with the local hard-soft acid-base principle. The results indicate that the intermediate formed from the electron-rich alkenes evolves preferentially toward the HT regioisomer whereas electron-poor alkenes tend to form the HH isomer, in agreement with experiment and previous theoretical studies.