Secondary erythrocytosis may arise from several causes, but an association with oxygen transport is rare. We describe for the first time a form of secondary erythrocytosis caused by compound heterozygosity for hemoglobin (Hb) Tak and (δβ)⁰-thalassemia found in an adult Thai individual. The patient had marked erythrocytosis and microcytosis with increased Hb and hematocrit values. Hb analyses using the Hb Gold Analyzer showed Hb A2 (72.5%) and Hb F (30.0%) without Hb A while the capillary electrophoresis revealed 2.3% Hb A2 and a major peak of Hb F (91.2%). Further molecular investigation identified that he was in fact a compound heterozygote for Hb Tak and deletional (δβ)⁰-thalassemia. Hematological parameters of the patient were compared with those observed for a Thai boy who demonstrated features of erythrocytosis and microcytosis caused by homozygous Hb Tak with α+-thalassemia and with those of pure carriers of Hb Tak and (δβ)⁰-thalassemia in our series. This report confirms the importance of both Hb and molecular investigations for the assessment of genotype/phenotype correlation and the appropriate management of the patients.