diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ebb19e6..04dffdc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         operating-system: ['ubuntu-latest']
-        php-versions: ['8.0', '8.1']
+        php-versions: ['8.1']
         phpunit-versions: ['latest']
     steps:
       - name: Checkout
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 723daeb..acd5aa8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,14 @@
 # Changelog
 
+## Version 5.1.0 (2202-05-23)
+
+* Dropped PHP 8.0 support, increased minimum PHP version to 8.1.
+  * This is due to the significant performance difference between ext/sodium
+    and sodium_compat, and the functions we use in 5.x aren't available until
+    PHP 8.1. See [#178](https://github.com/paragonie/halite/issues/178).
+* The 5.0.x branch will continue to *function* on PHP 8.0 but performance is
+  not guaranteed.
+
 ## Version 5.0.0 (2022-01-19)
 
 * Increased minimum PHP version to 8.0.
diff --git a/README.md b/README.md
index fc08b52..143543c 100644
--- a/README.md
+++ b/README.md
@@ -36,14 +36,17 @@ versions of Halite are briefly highlighted below.
 
 |                                                              | PHP   | libsodium | PECL libsodium | Support                   |
 |--------------------------------------------------------------|-------|-----------|----------------|---------------------------|
-| Halite 5.0 and newer                                         | 8.0.0 | 1.0.18    | N/A (standard) | :heavy_check_mark: Active |
+| Halite 5.1 and newer                                         | 8.1.0 | 1.0.18    | N/A (standard) | :heavy_check_mark: Active |
+| Halite 5.0.x                                                 | 8.0.0 | 1.0.18    | N/A (standard) | :heavy_check_mark: Active |
 | [Halite 4.1+](https://github.com/paragonie/halite/tree/v4.x) | 7.2.0 | 1.0.15    | N/A (standard) | :x: Not Supported         |
 | [Halite 4.0](https://github.com/paragonie/halite/tree/v4.0)  | 7.2.0 | 1.0.13    | N/A (standard) | :x: Not Supported         |
 | [Halite 3](https://github.com/paragonie/halite/tree/v3.x)    | 7.0.0 | 1.0.9     | 1.0.6 / 2.0.4  | :x: Not Supported         |
 | [Halite 2](https://github.com/paragonie/halite/tree/v2.2)    | 7.0.0 | 1.0.9     | 1.0.6          | :x: Not Supported         |
 | [Halite 1](https://github.com/paragonie/halite/tree/v1.x)    | 5.6.0 | 1.0.6     | 1.0.2          | :x: Not Supported         |
 
-If you need a version of Halite before 5.0, see the documentation relevant to that
+Note: Halite 5.0.x works on PHP 8.0, but performance is worse than on PHP 8.1.
+
+If you need a version of Halite before 5.1, see the documentation relevant to that
 particular branch.
 
 **To install Halite, you first need to [install libsodium](https://paragonie.com/book/pecl-libsodium/read/00-intro.md#installing-libsodium).**
diff --git a/composer.json b/composer.json
index f159981..c32fea2 100644
--- a/composer.json
+++ b/composer.json
@@ -32,7 +32,7 @@
         }
     ],
     "require": {
-        "php": "^8",
+        "php": "^8.1",
         "ext-json": "*",
         "paragonie/constant_time_encoding": "^2",
         "paragonie/hidden-string": "^1|^2",