Born on December 6, 1778, Joseph was the oldest of five children. His father, Antoine Gay, was a lawyer who, to distinguish himself from other people in the Limoges region with the last name of Gay, used the surname Gay-Lussac from the name of some family property near St Leonard. Poor guy.

           Gay-Lussac was sent to Paris at the age of fourteen when his father was arrested during the French Revolution. After having  private lessons and attending a boarding school (the Ecole Polytechnique and the civil engineering school) Gay-Lussac became an assistant to Berthollet who was himself a co-worker of Lavoisier, a famous french scientist. Gay-Lussac thus got the chance to become part of the group of famous men who spent time at Berthollet's country house near Arcueil. Here among the Arcueil Society he received his training in chemical research.

      With the encouragement of Berthollet and LaPlace, Gay-Lussac at the age of 24 conducted his first major research in the winter of 1801-1802. He settled some conflicting evidence about the expansion properties of different gases. By excluding water vapor from the apparatus and by making sure that the gases themselves were free of moisture, he obtained results that were more accurate than had been obtained previously by others. He concluded that equal volumes of all gases expand equally with the same increase in temperature. While Jacques Charles discovered this volume-temperature relationship fifteen years earlier, he had not published it. Unlike Gay-Lussac, Charles did not measure the coefficient of expansion. Also, because of the presence of water in the apparatus and the gases themselves, Charles obtained results that indicated unequal expansion for the gases that were water soluble.