diff --git a/src/lib/isBIC.js b/src/lib/isBIC.js
index 240bfe18b..b5576b24e 100644
--- a/src/lib/isBIC.js
+++ b/src/lib/isBIC.js
@@ -9,7 +9,7 @@ export default function isBIC(str) {
 
   // toUpperCase() should be removed when a new major version goes out that changes
   // the regex to [A-Z] (per the spec).
-  if (CountryCodes.indexOf(str.slice(4, 6).toUpperCase()) < 0) {
+  if (!CountryCodes.has(str.slice(4, 6).toUpperCase())) {
     return false;
   }
 
diff --git a/src/lib/isISO31661Alpha2.js b/src/lib/isISO31661Alpha2.js
index e91ae8717..e67bb1e15 100644
--- a/src/lib/isISO31661Alpha2.js
+++ b/src/lib/isISO31661Alpha2.js
@@ -1,7 +1,7 @@
 import assertString from './util/assertString';
 
 // from https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
-const validISO31661Alpha2CountriesCodes = [
+const validISO31661Alpha2CountriesCodes = new Set([
   'AD', 'AE', 'AF', 'AG', 'AI', 'AL', 'AM', 'AO', 'AQ', 'AR', 'AS', 'AT', 'AU', 'AW', 'AX', 'AZ',
   'BA', 'BB', 'BD', 'BE', 'BF', 'BG', 'BH', 'BI', 'BJ', 'BL', 'BM', 'BN', 'BO', 'BQ', 'BR', 'BS', 'BT', 'BV', 'BW', 'BY', 'BZ',
   'CA', 'CC', 'CD', 'CF', 'CG', 'CH', 'CI', 'CK', 'CL', 'CM', 'CN', 'CO', 'CR', 'CU', 'CV', 'CW', 'CX', 'CY', 'CZ',
@@ -27,11 +27,11 @@ const validISO31661Alpha2CountriesCodes = [
   'WF', 'WS',
   'YE', 'YT',
   'ZA', 'ZM', 'ZW',
-];
+]);
 
 export default function isISO31661Alpha2(str) {
   assertString(str);
-  return validISO31661Alpha2CountriesCodes.indexOf(str.toUpperCase()) >= 0;
+  return validISO31661Alpha2CountriesCodes.has(str.toUpperCase());
 }
 
 export const CountryCodes = validISO31661Alpha2CountriesCodes;