Crystals of challacolloite, KPb₂Cl₅, and hephaistosite, TlPb₂Cl₅, from volcanic sublimates formed on the crater rim of the “La Fossa Crater” at Vulcano, Aeolian Archipelago, Italy, were investigated. Chemical compositions were ( \textK_0.93 \textTl_0.02 )_{S = 0.95} \textPb_2.04 ( \textCl_4.90 \textBr_0.11 )_{S = 5.01} {\left( {{\text{K}}_{{0.93}} {\text{Tl}}_{{0.02}} } \right)}_{{\Sigma = 0.95}} {\text{Pb}}_{{2.04}} {\left( {{\text{Cl}}_{{4.90}} {\text{Br}}_{{0.11}} } \right)}_{{\Sigma = 5.01}} and \textTl_0.94 \textPb_2.01 ( \textCl_4.91 \textBr_0.14 )_{S = 5.05} {\text{Tl}}_{{0.94}} {\text{Pb}}_{{2.01}} {\left( {{\text{Cl}}_{{4.91}} {\text{Br}}_{{0.14}} } \right)}_{{\Sigma = 5.05}} , respectively. Single crystal X-ray measurements showed monoclinic symmetry for both phases, space group P2₁/c, with the following unit-cell parameters: a = 8.8989(4), b = 7.9717(5), c = 12.5624(8) Å, β = 90.022(4)°, V = 891.2(1) ų, Z = 4 (challacolloite) and a = 9.0026(6), b = 7.9723(6), c = 12.5693(9) Å, β = 90.046(4)°, V = 902.1(1) ų, Z = 4 (hephaistosite). The structure refinements converge to R = 3.99% and R = 3.86%, respectively. The effects of Br↔Cl and K↔Tl substitutions on the structure of these natural compounds have been discussed.