An electrochemical gas sensor package comprising: a housing defined by a lid or package body and a package substrate; an integrated device die comprising a sensor portion having a sensing element configured to detect a gas, and one or more gas channels formed through the integrated device die, the sensor portion being in fluid communication with the one or more gas channels; a die cap mounted to the integrated device die, the die cap at least partially defining a first chamber; an electrolyte in the first chamber; an outer chamber, defined by the housing, the integrated device die mounted to the package substrate of the housing within the outer chamber using die supports or standoff structures; a second chamber, at least partially defined by the package substrate, the die support or standoff structures and the integrated device die, the second chamber being in fluid communication with the outer chamber via openings in the die supports or lateral channels in the standoff structures, the first chamber being fluidly separate from the second chamber; a gas inlet to provide fluid communication between the outer chamber and the outside environs, the gas inlet configured to permit the gas to enter the outer chamber from the outside environs; and an additional integrated device die or electronic component mounted to the package substrate; wherein the sensor portion of the integrated device die has a first side at least partially exposed to the first chamber and a second side at least partially exposed to the second chamber, the first side opposite the second side; and wherein, in use, gas passes from the outside environs through the gas inlet into the outer chamber, then passes laterally through the openings or channels in the die supports or support structures into the second chamber, then through the one or more gas channels to impinge on the sensing element. a housing defined by a lid or package body and a package substrate; an integrated device die comprising a sensor portion having a sensing element configured to detect a gas, and one or more gas channels formed through the integrated device die, the sensor portion being in fluid communication with the one or more gas channels; a die cap mounted to the integrated device die, the die cap at least partially defining a first chamber; an electrolyte in the first chamber; an outer chamber, defined by the housing, the integrated device die mounted to the package substrate of the housing within the outer chamber using die supports or standoff structures; a second chamber, at least partially defined by the package substrate, the die support or standoff structures and the integrated device die, the second chamber being in fluid communication with the outer chamber via openings in the die supports or lateral channels in the standoff structures, the first chamber being fluidly separate from the second chamber; a gas inlet to provide fluid communication between the outer chamber and the outside environs, the gas inlet configured to permit the gas to enter the outer chamber from the outside environs; and an additional integrated device die or electronic component mounted to the package substrate; wherein the sensor portion of the integrated device die has a first side at least partially exposed to the first chamber and a second side at least partially exposed to the second chamber, the first side opposite the second side; and wherein, in use, gas passes from the outside environs through the gas inlet into the outer chamber, then passes laterally through the openings or channels in the die supports or support structures into the second chamber, then through the one or more gas channels to impinge on the sensing element.